ABUNDANCE, REPRODUCTION, AND PREY OF RHINOCEROS AUKLET,
CERORHINCA MONOCERATA, ON ANO NUEVO ISLAND, CALIFORNIA
A thesis submitted to the faculty of San Francisco
State University and Moss Landing Marine
Laboratories in partial fulfillment
of the requirements for
the degree
Masters of Science
m
Marine Science
by
Michelle M. Hester
Moss Landing, California
October, 1998
Copyright by
Michelle M. Hester
1998
ABUNDANCE, REPRODUCTION, AND PREY OF RHINOCEROS AUKLET,
CERORHINCA MONOCERATA, ON ANO NUEVO ISLAND, CALIFORNIA
Michelle M. Hester
San Francisco State University, Moss Landing Marine Laboratories
1998
Annual size of the breeding population of Rhinoceros Auklets on Aiio Nuevo
Island, California, was estimated from 1993 to 1995. Occupancy rates were derived by
monitoring nest cavities using a miniature burrow camera. From 1993 to 1995, the number
of breeding birds increased 69% (102 to 172 birds). To determine reproductive
performance, pairs breeding in nest boxes and natural burrows were monitored. Timing of
reproduction was significantly earlier in 1995. During the first two years after nest box
installation, productivity was low; only 33% of the breeding pairs produced independent
offspring. Productivity in nest boxes increased in 1995, but was significantly less than that
attained by pairs in natural burrows. Chick growth rates (linear phase 10-40 days) ranged
from 7.0 to 10.6 g/day and were significantly greater in 1995. Prey collected from adults
delivering meals to chicks indicated Rhinoceros Auklets fed mostly northern anchovy,
Engrau/is mordax, to their chicks each year. Weights of bill-loads did not significantly
differ within or among years. Prey availability was investigated by summarizing National
Marine Fisheries Service mid-water trawl data from stations near Aiio Nuevo Island in
1993 and 1994. This comparative analysis indicated that adults in general did not feed
chicks the most abundant resources in their foraging area and chick provisioning may be
significantly affected if local northern anchovy stocks decline.
I certify that the Abstract is a correct representation of the content of this thesis.
ACKNOWLEDGMENTS
Financial support for this study was provided by the State of California Department
of Parks and Recreation, Bay Area District, Point Reyes Bird Observatory (PRBO), and
the Earl H. Myers and Ethel M. Myers OcealJl)graphic and Marine Biology Trust.
I thank the members of my graduate advisory committee, Dr. James T. Harvey,
William J. Sydeman, Dr. Gregor Cailliet, and Dr. Sarah G. Allen. Special appreciation to
William Sydeman for his guidance over the years.
I am indebted to Gary Strachen, supervising ranger at Aiio Nuevo State Reserve
(ANSR), for supporting PRBO and myself in our efforts to protect and monitor the seabird
populations on Aiio Nuevo Island. Paul Keel, Bern Smith, and other staff at ANSR also
contributed immensely to the initiation of this study.
I gratefully thank National Marine Fisheries Service, Tiburon Laboratory,
especially Steve Ralston, for providing the midwater trawl data used in this study.
Over 50 colleagues and friends from PRBO, MLML, Granite Canyon, and UCSC
helped me in the field from 1993 to 1995. Sincere thanks for lifting the zodiac, hauling
gear, digging in the sand, getting bit by rhinos and shat on by gulls, and making these three
years fun and successful.
I would like to thank Jerry Nusbaum for building the nest boxes and the Volunteer
Telephone Pioneers and Big Creek Lumber for providing time, craftsmanship, and supplies
for the boardwalk project.
I conducted this research as an employee ofPRBO and am thankful to be part of an
organization made up of biologists and staff committed to understanding and preserving
wildlife populations. I thank PRBO for continuing to support this research, hopefully long
after my involvement.
I would like to express my gratitude to my friends at Granite Canyon, especially
Brian Anderson and John Hunt, who always supported me and put up with the distractions
of my thesis while I was employed there.
My deepest thanks to Steven Osborn who's endless encouragement, patience, and
advice kept me afloat through this process.
v
TABLE OF CONTENTS
Page
List of Tables .................................................................................................................... vii
List of Figures .................................................................................................................. viii
Introduction ......................................................................................................................... I
Materials and Methods ......................................................................................................... S
Study Site................................................................................................................ .5
Nest Box Installation ................................................................................................ 5
Breeding Population Size......................................................................................... 6
Reproductive Performance ....................................................................................... 7
Prey Composition and Biomass ............................................................................... 9
Prey Availability .................................................................................................... lO
Results ................................................................................................................................ 12
Breeding Population Size....................................................................................... 12
Reproductive Performance..................................................................................... 12
Prey Composition and Biomass ............................................................................. 14
Prey Availability .................................................................................................... 16
Discussion .......................................................................................................................... !?
Breeding Population Size....................................................................................... I?
Reproductive Performance ..................................................................................... 19
Prey Composition and Biomass ............................................................................. 22
Prey Availability .................................................................................................... 23
Summary and Conclusions ................................................................................................. 29
Literature Cited .................................................................................................................. 31
Tables ................................................................................................................................. 38
Figures ............................................................................................................................... .44
VI
LIST OF TABLES
Table
Page
1. Summary of Rhinoceros Auklet breeding population estimates on
Afio Nuevo Island.
38
2. Timing of reproduction of Rhinoceros Auklets on Afio Nuevo Island
39
3. Growth rates of Rhinoceros Auklet chicks (during the linear growth phase)
reared in nest boxes on Afio Nuevo Island.
40
4. Reproductive performance of Rhinoceros Auklet pairs breeding in nest boxes
and natural burrows on Afio Nuevo Island.
41
5. Weight of bill-loads delivered to Rhinoceros Auklet chicks by standardized
sample date and year.
42
6. Occurrence of prey species in Rhinoceros Auklet chick meals and in
Pescadero/Davenport trawls.
43
Vll
LIST OF FIGURES
Figure
Page
1. Map of Aiio Nuevo Island showing Rhinoceros Auklet breeding areas (1-6).
44
2. Map showing National Marine Fisheries Service trawl stations used in this study. 45
3. Distribution of Rhinoceros Auklet egg-laying dates on Afio Nuevo Island.
46
4. Percent occurrence and percent biomass of prey species in Rhinoceros Auklet
chick diet for each netting date in 1993.
47
5. Percent occurrence and percent biomass of prey species in Rhinoceros Auk let
chick diet for each netting date in 1994.
48
6. Percent occurrence and percent biomass of prey species in Rhinoceros Auk let
chick diet for each netting date in 1995.
49
7. Mean length of each fish species in Rhinoceros Auklet chick diets.
50
8. Distribution and abundance of northern anchovy, Engraulis mordax,
in trawls along Davenport and Pescadero transects.
51
9. Distribution and abundance of market squid, Lo/igo opolescens, in trawls
along Davenport and Pescadero transects.
52
10. Distribution and abundance of juvenile shortbelly rockfish, Sebaste.1·jortlani,
in trawls along Davenport and Pescadero transects.
53
11. The proportion of species in the chick diets compared with the proportion
in the trawls in 1993.
54
12. The proportion of species in the chick diets compared with the proportion
in the trawls in 1994.
55
13. Rhinoceros Auklet population trends on Aiio Nuevo Island.
56
14. Rhinoceros Auklet productivity on Aiio Nuevo Island and Southeast
Farallon Island.
57
Vlll
INTRODUCTION
The Rhinoceros Auklet (Cerorhinca monocerata), a seabird in the family Alcidae,
breeds throughout temperate waters of the North Pacific Ocean. Despite their common name,
they are more closely related to puffins than auklets based on morphology, life history, and
systematics (Storer 1945, Hudson eta/. 1?69, Strauch 1988). In the spring, Rhinoceros
Auklets leave their off-shore foraging grounds and return to island colonies to breed. World
population estimates are roughly 1-3 million individuals.
In California, Rhinoceros Auklets were once plentiful, but the population began to
decline in the early 1800's (Heermann 1859, Gruber 1884). By 1865, the breeding
population had disappeared (Grinnell 1926). Studying this burrowing, nocturnal species was
difficult, therefore, reasons for their disappearance cannot be determined. After an absence of
more than 100 years, a natural recolonization event began in the 1970's. Three islands, Castle
Rock (41 °N, 124°W), the Farallon Islands (37°N, 123°W), and Ana Nuevo Island (37°N,
122°W), provide nesting habitat for approximately 90% of the California breeding population
(Carteret a/. 1992). Currently, expansions have been documented as far south as San Miguel
Island (34°N, 120°W; McChesney eta/. 1995). In 1989-1991, the estimated breeding
population in California was roughly 1,800 birds (Carteret a/. 1992).
Range expansion or contraction may be influenced by changing environmental
conditions on several scales, from climate change to quality of nesting habitat. In California,
possible factors facilitating Rhinoceros Auklet recolonization are: 1) reduced human
disturbance; 2) reduced competition for nest sites; and 3) adequate food resources.
Individuals recruiting to these new colonies could be juveniles or adults from nearby breeding
colonies and wintering areas. Briggs eta!. (1987) estimated roughly 300,000 individuals off
California during winter, indicating movement from northern areas to California.
Factors that regulate Rhinoceros Auklet population dynamics are poorly understood.
Survivorship, recruitment, and life-time reproductive success have not been investigated.
1
However, studies have been conducted on breeding biology and food habits in Washington
(Leschner 1976, Richardson 1961, Wilson and Manuwal1986), British Columbia (Summers
and Drent 1979, Vermeer 1980, Vermeer and Westrheim 1982, Vermeer and Devito 1986,
Bertram and Kaiser 1993, Harfenist 1995), and southeast Alaska (Hatch 1982). In
California, research conducted on Southeast Farallon Island since 1986 is the only ecological
information available for this species (Ainley and Boekelheide 1990, Point Reyes Bird
Observatory [PRBO] unpubl. data). Recently, researchers have identified factors that cause
mortalities and affect reproductive efforts, such as predation (Piatt eta/. 1988, Bailey 1990,
Paine 1990), kleptoparasitism (Watanuki 1990, Wilson 1993, Harfenist and Ydenberg 1995,
Miyazaki 1996), gill-netting (Ainley eta/. 1981, DeGange and Day 1991 ), and oil spills (Page
eta/. 1990, Ford eta/. 1991). Recolonizing and expanding Rhinoceros Auklet populations in
California may utilize different resources, habitat types, and encounter different threats than
established northern colonies.
As for all long-lived species, monitoring during many years is necessary to understand
how populations are regulated. This endeavor is complicated for Rhinoceros Auklets because
censusing is problematic. Population estimates are usually unreliable because of difficulty in
determining burrow occupancy. Nests are located at the end of burrows or crevices (1-4m
long), usually past arms reach and out of sight. Traditional methods of monitoring occupancy
involve using entrance indicators (sticks placed upright in tunnels) or excavating burrows.
The first method is unreliable because Rhinoceros Auklet behavior and attendance patterns on
the colony is usually not known. The second method results in habitat damage and
disturbance to birds. On Protection Island, Washington, 28-30% of burrows occupied during
incubation were deserted after excavations (Wilson 1977). Utilizing surveillance technology,
new methods have been developed to reduce disturbance. Researchers have used miniature
burrow cameras to monitor nest cavities ofprocellariids (Dyer and Hill 1991, Ainley et al.
1995) and Burrowing Owls (Rosenberg eta/. 1997). This method could significantly
2
improve Rhinoceros Auklet research and result in more accurate population estimates.
The Rhinoceros Auklet population on Ana Nuevo Island, California, presents an
opportunity to document annual fluctuations in population size while investigating regulatory
factors such as reproductive performance in relation to prey availability. Because the colony
is small (< 200 birds) and accessible, the entire breeding population can be censused. Past
censuses indicated the population grew between 1987 and 1989 (Carteret a/. 1992).
However, breeding habitat on this island is threatened by erosion and disturbance (Hester and
Sydeman 1996) and may be limited. Accurate annual estimates of population size will indicate
how the colony responds to this dynamic habitat.
Measurements of reproductive performance (i.e. productivity, timing of reproduction,
chick growth) are fundamental to understanding population dynamics. To aid in monitoring
breeding activities, nest boxes have been installed on other colonies (Wilson 1986, Ainley and
Boekelheide 1990), although use of the boxes by breeding birds varied. A pilot study
conducted by Lewis and Tyler (1987) indicated Rhinoceros Auklets on Ana Nuevo Island
occupied nest boxes within the first year of installation. However, no study has verified that
pairs breeding in artificial sites are representative of those in natural sites.
In response to reductions or changes in food availability, breeding seabirds may alter
parental time budgets, switch food resources, or abandon reproductive efforts (Martin 1989,
Burger and Piatt 1990, Hamer et al. 1993, Decker eta/. 1995, Hodder and Graybilll995).
Researchers have suggested that the Rhinoceros Auklet is more efficient at adapting to prey
fluctuations than other piscivorous seabirds. In 1992, El Nino effects altered prey
distributions near the Farallon Islands, which resulted in near total reproductive failure of the
Common Murre (Uri a aa/ge), Pelagic Cormorant (Pha/acrocorax pelagicus), and Brandt's
Cormorant (Phalacrocorax penicillatus; McLaren eta/. 1992). The Rhinoceros Auklet
population breeding on the same colony fledged 8.4% more chicks than average in 1992
(PRBO unpubl. data). In addition, since 1987, Rhinoceros Auklet chicks on Southeast
3
Farallon Island were fed 21 different prey species (PRBO unpubl. data). This ability to utilize
alternative prey resources may allow breeding to continue when traditional prey stocks are
depleted.
Seabirds are restricted to a variable foraging radius around the colony. The nearshore
oceanic environment surrounding Afio Nuevo Island (1 km offshore) is more homogeneous
than that of the Farallon Islands (42 km offshore near the shelf break). The diversity of prey
species may be less nearshore and Afio Nuevo Island breeders may respond differently to the
same environmental anomalies than those breeding offshore.
The vulnerability of prey depends on the predators ability to locate and capture the
prey. Depth below the surface, daily migration patterns, schooling behaviors, abundance,
and other factors affect prey vulnerability. Since 1983, the National Marine Fisheries Service
has conducted midwater trawl surveys, targeting fishes at several early life stages (Adams
1992). These trawls were conducted at night when most pelagic young-of-the-year fish
migrate towards the surface. Rhinoceros Auklets prey on the pelagic fish populations targeted
in the above surveys. By monitoring Rhinoceros Auklet chick diet and the abundance and
distribution of food near Afio Nuevo Island, local predator-prey relationships can be studied.
Understanding this relationship can lead to better management of local fish stocks and
Rhinoceros Auklet populations.
The goal of this study was to estimate breeding population size and investigate
reproductive performance and prey utilization of the newly colonized population of
Rhinoceros Auklets on Afio Nuevo Island from 1993 to 1995. The main objectives were to:
1) estimate annual breeding population size; 2) monitor breeding activities in nest boxes and
natural burrows to determine productivity, timing of reproduction, and chick growth rates; 3)
quantify prey composition and biomass of chick diets; and 4) investigate relationships
between chick diet and the distribution and abundance of prey in the environment using
mid water trawl data.
4
MATERIALS AND METHODS
Rhinoceros Auklets are sensitive to disturbance caused by researchers. Extreme
caution was taken in conducting this research. Field techniques used were previously
tested on the Farallon Island colony (PRBO unpubl. data). Statistical analyses were
performed using the program STATA v.3.1 and significance was assumed ifp<0.05.
Study Site
Aiio Nuevo Island (37° 06' 30" N, 122° 20' 09" W) is a small coastal island
separated from the mainland by approximately 1 km of water. The island is managed by
the State of California Parks and Recreation Department and is not accessible to the public.
The island is comprised of a marine terrace, of approximately 3.6 hectares, with sandy
beaches and wave swept islets. The maximum elevation is approximately 9 m above sea
level. Rhinoceros Auklet burrows are located along the edges of the marine terrace in areas
1-6 (Fig. I). Burrows are found among vegetation, under boards, and most frequently in
dry loose sand. In spring, the dominant oceanographic process influencing the marine
ecosystem near this island is a coastal upwelling plume centered at Point Aiio Nuevo
(Rosenfeld eta/. 1994).
Nest Box Installation
Nest boxes were installed on Aiio Nuevo Island to protect and enhance breeding
habitat and aid in monitoring. Nest boxes (61 ern x 25 em x 25 em) were constructed with
half inch plywood and tunnels of polystyrene pipe (15 ern diameter) attached to one end of
the box. Natural burrow tunnels may range from 1-4m in length (pers. obs.). Artificial
tunnel lengths were chosen randomly within this range. Forty nest boxes were installed
underground in 1993 and 16 more in 1994. Sites for nest box installation were chosen
randomly in areas 1 - 5 where Rhinoceros Auklet burrows already existed (Fig. 1). To
5
simulate natural conditions, moist soil was packed inside the boxes and tunnels (to a depth
of about 5 em). The slope of artificial tunnels did not exceed 20 degrees; thus preventing
entrapment of birds in the nest boxes. Soil was packed on top of the box such that water
would drain away instead of pooling above the lid. Nest boxes were repaired each
February, before the breeding season, and !fiaintained as necessary. Nest boxes provided a
protected site that better withstood the effects of trampling and erosion. Adults, eggs, and
chicks could be handled in nest boxes with limited disturbance through a removable lid.
The boxes were available to newly recruiting birds or pairs whose old nest sites had been
destroyed through natural or unnatural events.
Breeding Population Size
The breeding population on Afio Nuevo Island nests in nest boxes and natural
burrows. Occupancy of nest boxes was evaluated by viewing inside the box through a
removable lid once every seven days in 1993 and once every five days in 1994 and 1995.
In addition to presence of eggs and chicks, a box was considered in use by a breeding pair
if fresh egg shell fragments were found. Movements of individually banded pairs between
nest boxes was documented and the population estimate adjusted accordingly.
To determine breeding occupancy of natural burrows, a miniature surveillance
camera with infrared illumination was designed and built. While viewing a monitor, the
camera lens was pushed down a burrow using a long cable. The camera was removed
from the burrow when either an egg, chick and/or adult was seen or the back of the burrow
was reached. No physical contact was made with the birds or eggs. A burrow was
considered occupied by one breeding pair if it contained an egg (whole or fragments),
chick, or if adults were viewed twice in an incubating posture.
6
Occupancy rate was defined as the percentage of successfully monitored burrows
(i.e. those in which the end was clearly viewed) occupied by breeding pairs. In 1993, an
occupancy rate was derived by determining the contents of inadvertently crushed burrows.
In 1994 and 1995, 50 burrows were randomly selected and checked for occupancy
approximately once every 10 days. Some.burrows were inaccessible due to obstructions
(e.g. roots, multiple turns) or the position on cliffs. In 1994, monitoring of burrows did
not begin until June; therefore, early-laying pairs that lost eggs or young chicks may have
been missed causing an underestimation of the occupancy rate. Once every 10 days natural
burrows were mapped and whether the entrance was open or closed was recorded.
Throughout the entire season, construction of new burrows was begun by adults (probably
non-breeders) and subsequently abandoned. To estimate the breeding population, annual
occupancy rates were applied to the total number of bun·ows consistently open from mid
April through June in areas 1-5 each year.
Size of the breeding population was calculated each year for areas 1-5. Number of
pairs breeding in boxes plus the estimate of pairs breeding in burrows was the total
breeding population. Area 6 was not monitored to avoid disturbing nesting Brandt's and
Pelagic Cormorants. The number of open burrows in area 6 before and after the breeding
season was documented to compare among years. These numbers were not added to areas
1-5 because they do not represent the breeding population; adults may dig new burrows late
in the season and burrows of pairs that failed to fledge a chick may disappear.
Reproductive Performance
To determine timing of reproduction, chick growth, and productivity, breeding
activities were monitored in nest boxes from 1993 to 1995. Beginning in April, nest boxes
were checked once every seven days in 1993 and once every five days in 1994 and 1995.
7
Bill depth and hom length of incubating adults and length and width of eggs were
measured to the nearest 0.1 mm. Adults and eggs were weighed to the nearest 1 g. Adults
and chicks were individually marked with stainless steel leg bands. Once an egg was
found, the site was left undisturbed until the estimated hatch date (approximately 42 days).
To determine timing of reproduction, the approximate dates of egg-laying,
hatching, and fledging were recorded to the accuracy of the observation frequency . In
1995, due to ocean conditions, nest boxes were not checked on 10 June or 15 June. This
was the peak hatching period; therefore, when necessary, hatch dates were estimated by
assuming chicks hatched 46 days (the average incubation period for 1995) after egg laying.
Mean egg-laying dates (converted to julian dates) were compared among years using an
analysis of variance (ANOV A) followed by a multiple-comparison test (Bonferroni).
Rhinoceros Auklets generally produce a single egg per year, although pairs can
re-lay if the first attempt was tenninated early in the season. The term "re-lay" was used to
describe a replacement egg produced by the same pair (confirmed by reading band
numbers) after the original egg had disappeared or was found dead (i.e. cold, cracked,
addled). Sometimes a second egg of unknown origin was found in a nest. These eggs,
tem1ed "second" eggs, could have been laid by a female that was subsequently evicted by
the resident pair or an unviable egg laid by the resident female.
Growth rates were calculated for all chicks in nest boxes. Adults brood their chicks
for approximately five days, thereafter, chicks are unattended during the day. To determine
growth rates, each chick was weighed (to the nearest 1 g) between 1100 and 1400 h.
Growth curves are sigmoidal for Rhinoceros Auklets ( Leschner 1976, Vermeer and Cullen
1979, Harfenist andY denberg 1995, Hester and Sydeman 1996). Growth rates (g/day)
were calculated for the linear phase of growth (10-40 days). Mean chick growth rates were
compared among years using an analysis of variance (ANOV A) followed by a multiple-
8
comparison test (Bonferroni). Mean adult weight, from those in nest boxes and mist nets
(see below), was compared among years using an analysis of variance (ANOVA).
Productivity of pairs breeding in nest boxes was determined each year.
Productivity was defined as the number of fledglings produced per breeding pair.
Rhinoceros Auklet pairs can rear a maximum of one chick per year; thus productivity
ranged from 0 to 1.0. When a chick is ready to fledge, it departs the nest site sometime
during the night. A chick was assumed to have successfully fledged if it was observed
"fully-feathered" before disappearance. Mean productivity of pairs in nest boxes was
compared among years using a chi-square analysis.
In 1995, the burrow camera was used to estimate productivity in natural burrows.
Monitoring of each burrow was attempted three times; once during incubation; hatching,
and chick-rearing. To estimate productivity it was assumed that a successful fledging
occurred if a chick was observed on or after the earliest recorded fledge date. To avoid
over estimation because buried eggs and/or shell fragments may not have been detected, it
was assumed all burrows contained a breeding pair. Additionally, to determine timing of
reproduction, a subsample of20 natural burrows was monitored every five days in 1995.
This field technique had not been tested previously. To control for disturbance, another
subsample of 20 burrows was monitored only twice; once during peak incubation and once
during chick rearing period. Mean productivity of pairs in disturbed and "less" disturbed
burrows were compared using a chi-square analysis. Mean productivity of pairs breeding
in nest boxes and natural burrows was compared using a chi-square analysis.
Prey Composition and Biomass
To quantify prey composition, diet items were collected from adults captured in
mist nets (2.1 m x 12.6 m, 6.0 em mesh) placed in areas 1 and 2 (Fig. 1). Adults carry
9
whole prey items crosswise in their bill as they return to feed their chicks. Prey are
dropped upon flying into nets. The term "bill-load" was used to describe all prey item(s)
dropped by one adult. During the chick rearing period, four netting sessions were
conducted for three hours beginning at dusk. The four sampling dates varied among years
depending on breeding chronology. Diet
s~mpling
deprives chicks of food. Richardson
(1961), however, found that each parent feeds its chick at least once each night, and may
arrive any time between dusk and dawn. Therefore, a chick of a captured bird may have
recieved food after sampling the same night. At most, each chick of a captured bird was
deprived of food on four nights during a 50 day nestling phase. Whole fish were identified
and weighed to the nearest 0.1 g. Fork length and standard length of fishes were measured
to the nearest 0.1 mm. Fish pieces were weighed and identified when possible. The
identification of some samples were verified by biologists at Moss Landing Marine
Laboratories, California, and the Tiburon Laboratory of the National Marine Fisheries
Service, California. To determine accurate weights of decomposed fish (due to sample
handling), existing length-weight regressions were used for northern anchovy, Engraulis
mordax (wt (g)= 3.58 x SL 2.74, Messersmith 1969), Pacific herring, Clupea prillosi (wt
(g)= 3.97 x FL 3.2, Spratt 1981, Vermeer and Devito 1986), and Pacific sardine,
Sordinops sogax (wt (g)= 5.40 x SL 3.15, Clark 1928).
Differences in mean standard lengths of the dominant prey species was tested
within a season and among years using an analysis of variance (ANOV A) followed by a
multiple-comparison test (Bonfen·oni). Mean weight of bill-loads was compared within a
season and among years using an analysis of covariance (ANCOVA).
10
Prey Availability
National Marine Fisheries Service conducted midwater trawls in waters off central
California, including those surrounding Afio Nuevo Island, to estimate abundance of
pelagic young of the year rockfish (Adams 1992). These surveys used a standard 26 x 26
m modified Cobb midwater trawl, with a. cod-end liner of9.53 rnrn (3/8") stretched mesh.
At each station a 15-minute nighttime trawl sample was taken at standard depths of 10, 30,
or 100m. All items caught were identified and counted. The June 1993 and 1994 trawl
data were summarized for selected stations. Only trawls conducted in June overlapped
with chick diet sampling. Davenport and Pescadero trawl stations were analyzed, which
were within the assumed 60 km foraging range of Rhinoceros Auklets (Fig. 2). Trawl
stations (122- 126 and 130- 134, Fig. 3) were 10 krn to 43 km from Aiio Nuevo Island.
The abundance and distribution offish and cephalopod species were determined for
each trawl. Species fed to chicks and caught in trawls were defined as "co-occurring
species." To investigate distributional patterns, the occurrence along transects was plotted
for some co-occurring species. The proportions of fish and cephalopod species in the
chick diets were compared with the proportions in trawls.
Tv lev's (1961) electivity index was used to measure usage and availability of food
resources. Electivity values (E) were calculated for species that occurred in chick diets
and/or made up greater than I% of the trawl catches ( Ei = ri- ni I fi + ni. where fi is the
percentage of species i in the diet and ni is the percentage of species i in the environment).
A "utilized" prey species is defined as one which occurred in the chick diet at a higher
propmtion than in the environment (trawls). Electivity values ranged from 0 to + 1.0
indicating usage and 0 to -1.0 indicating non-usage.
11
RESULTS
Breeding Population Size
From 1993 to 1995 the estimated number of breeding birds increased 69% (102 to
172 birds; Table 1). The annual rate of increase (A.) was 1.49 from 1993 to 1994 and 1.13
from 1994 to 1995. The percentage ofne~t boxes occupied by breeding pairs increased
from 30% in 1993 to 57% in 1995. Between 1993 and 1994 the number of natural
burrows increased, then slightly decreased in 1995 (Table 1). The percentage of burrows
occupied by breeding pairs increased from 1993 to 1995 (Table 1). In 1993, of 11
burrows crushed, 73% (8) contained a breeding pair. In 1994, occupancy was assessed
(miniature camera method) in 50 of the 69 burrows. Seven of these burrows were
excluded because they were not censused until after 14 July, therefore chicks may have
been missed that had already fledged. Of the resulting 43 burrows, 81% (35) contained a
breeding pair. Occupancy was assessed in 52 of the 64 burrows in 1995. Of these, 85%
(44) contained a breeding pair. Burrows in area 6 were not included in the population
estimates, although in this area there were 14 burrows in April1994 and 10 burrows in
March 1995.
Reproductive Performance
Rhinoceros Auklets were present on Afio Nuevo Island from January through
August. In January and February, carcasses from recent raptor kills and open burrows
with fresh dig marks were present. Adults were found attending sites during the day as
early as 17 March (Table 2). From 1993 through 1995, the breeding season spanned from
mid April to mid August; earliest egg laying was 17 April and the latest fledging was 20
August (Table 2). Non-breeding birds were attending the colony during and after the chick
period; as indicated by newly dug burrows. New burrows were found as late as
September each year.
12
Timing of egg-laying varied significantly among years (F=4.04, P=0.022; Table 2)
and was earlier in 1995 than 1993 (Bonferroni test, P=0.028). Adults that laid eggs after
18 May 1993,5 May 1994, and 12 May 1995 did not produce independent offspring that
year (Fig. 3). One exception was a pair that re-layed on 31 March 1995, which
successfully fledged a chick.
On two occasions in 1995, a breeding pair transferred to another nest box andrelayed. "Second" eggs were found in six nests during this study. Of 43 eggs that did not
hatch (including re-lay and "second" eggs) 55.8% (24) were found cold, 34.9% (15) were
found cracked, 7.0% (3) disappeared, and 2.3% (1) died during hatching.
Chick growth, during the linear phase of growth, varied significantly among years
(F=6.80, P=0.005) and was significantly greater in 1995 than 1993 or 1994 (Bonferroni
test, P=0.005; Table 3). In 1994, the chick that did not successfully fledge began losing
weight at a rate of -4.0% per day, and was found outside its nest box wounded by a
Western Gull, as evidenced by characteristic skull punctures. The chick that did not
survive to fledge in 1994 began losing weight at a rate of -5.8% per day, and apparently
starved 30 days after hatch.
Poor hatching success accounted for all but two failed breeding attempts monitored
during this study (Table 4). Productivity of pairs in nest boxes was greater in 1995 than
1993 or 1994, although not significantly (X2=1.24, P=0.54; Table 4). In 1995, pairs in
natural burrows produced significantly more offspring than those in nest boxes
(X2=10.89, P=0.0015; Table 4).
Adults in natural burrows monitored for phenology were disturbed 6 to 10 times
and 92% of the chicks fledged (n=l2). Randomly selected "control" burrows were
disturbed only twice, and 100% of the chicks fledged (n=11). Productivity was similar
between these groups (X2=0.958, P=0.32).
13
Prey Composition and Biomass
From 1993 to 1995, 10 different prey species were collected from Rhinoceros
Auklet bill-loads. In 1993, the diet of chicks consisted of four species of fishes: northern
anchovy,juvenile shortbelly rockfish (Sebastesjordnni), Pacific sardine, and Pacific
herring. Of the 59 birds caught in mist nets, the percentage of birds carrying fish
decreased throughout the chick period with 44% on 10 July, 45% on 24 July, 27% on 31
July, and 25% on 7 August. Twenty bill-loads were collected which contained 95 fish.
Northern anchovy were dominant numerically followed by Pacific sardine, Pacific
herring, and shortbelly rockfish (Fig. 4). Northern anchovy were also dominant in terms
of biomass followed by Pacific sardine, shortbelly rockfish, and Pacific herring (Fig. 4).
Because gill characteristics are used to identify sardines from herrings (Miller and Lea
1972) and Rhinoceros Auk lets carry their prey just behind the head, 25% of the fish could
not be identified and were grouped as "sardlherr". Approximately 15% of the fish brought
to the chicks were damaged or consisted of unmeasurable pieces. Northern anchovies
remained the most important prey species throughout the chick period. Shortbelly
rockfish constituted 25% of the prey on 10 July but disappeared from the diet thereafter.
The relative importance of Pacific sardine and Pacific herring increased at the end of the
chick period, but sample sizes were small.
In 1994, chick meals sampled consisted of six species of fishes and one
cephalopod: northern anchovy, Pacific saury (Cololabis saira), shortbelly rockfish, Pacific
sardine, Pacific sandlance (Ammodytes hexapterus), and market squid ( LD/igo opalescens).
The percentage of birds (43) carrying fish varied throughout the chick period with 40% on
24 June, 85% on 4 July, 42% on 15 July, and 31% on 23 July. Twenty-five bill-loads
were collected which contained 91 fish and 8 cephalopods. Northern anchovies were
dominant numerically and by biomass, followed by Pacific saury, market squid, Pacific
sardine, shortbelly rockfish, and Pacific sandlance (Fig. 5). Pacific sardines were only
14
found in the beginning of the chick period, and the relative importance of market squid
increased at the end of the chick period (Fig. 5).
The diet of chicks in 1995 consisted of four genera of fishes and one cephalopod:
northern anchovy,juvenile lingcod (Ophiooon elongatus), juvenile rockfish (Sebastes
sp.), juvenile salmon (Oncorhynchus sp.), .and market squid. Of the 55 birds caught in
mist nets, the percentage of birds carrying fish generally decreased throughout the chick
period; 43% on 25 June, 50% on 5 July, 30% on 15 July, and 6% on 25 July. Seventeen
complete and 5 partial bill-loads were collected which contained 49 fish and 1 cephalopod.
Northern anchovies were dominant numerically and by biomass, followed by lingcod,
salmon sp., rockfish sp., and market squid (Fig. 6). Northern anchovies were the most
important prey species for each sampling period in terms of biomass but lingcod was
dominant numerically on 25 June (Fig. 6). Lingcod was not found in chick meals after 25
June (Fig. 6).
The amount of food delivered per bill-load was similar among years (F=0.45,
P=0.637; Table 5) and within each season (F=0.02, P=0.997; Table 5). Chick meals
ranged from 2.3 g to 75.7 g per bill-load (Table 5).
Fish prey in bill-loads ranged from 40 mm to 159 mm standard length (SL; Fig. 7).
Pacific herring and juvenile rockfish were the smallest prey delivered (less than 80 mm SL;
Fig. 7). The largest prey foraged upon were Pacific saury and juvenile salmon (Fig. 7).
Mean standard lengths of northern anchovy were significantly greater in 1995 ( x=123.5
mm, SE=4.1, n=34) than 1994 (x=63.3 mm, SE=5.6, n=68) and 1993 (x=70.6 mm,
SE=3.6, n=73; F=82.8, P<0.0001). Age-classes of northern anchovy were age 0-2 in
1993 and 1994 (SL=40-130 mm) and age 0-5 in 1995 (SL=106-153 mm); based on
length/age relationships reported for northern anchovies off central California (Parrish et al.
1983).
15
Prey Availability
National Marine Fisheries Service conducted 13 trawls in 1993 and 9 trawls in
1994 in the Davenpoii!Pescadero transects (Fig. 2). Pacific hake (Mer/uccius productus;
0.68) was the most abundant species in 1993 and larval northern anchovy (0.27) and
sanddabs (Citharichthys spp.; 0.20) were most abundant in 1994 (fable 6).
The dominant prey item, northern anchovy, comprised 2% of the trawl catch in
1993 and 10% in 1994 (not including larval stage; Table 6). Most northern anchovy
occurred nearshore in water less than 100m (Fig. 8). Pacific saury, the second most
abundant diet item, did not occur in any trawls. The third most abundant diet item, market
squid, comprised 5% of the trawl catch in 1993 and 12% in 1994. The majority of the
squid were caught in a I 0-m Pescadero trawl at 20m bottom depth (Fig. 9), however,
smaller numbers occurred throughout Davenport and Pescadero stations. The fourth most
abundant prey species in 1994, shortbelly rockfish, comprised 6% of the trawl catch. This
species was 1% of the trawl catch in 1993, but did not occur in chick diets that year. Most
rockfish were caught in a 10-m and a 30-m trawl off Pescadero at 170 m bottom depths
(Fig. 10).
In 1993, Pacific herring and Pacific sardine had an electivity index of 1.0 (Fig. 11).
Also, northern anchovy were brought to chicks in greater proportion than caught in trawls
(0.97; Table 6). Shortbelly rockfish was the only species in 1993 where usage as prey
approached the proportion in trawls. In 1994, species with an electivity index of 1.0
included Pacific sandlance, Pacific sardine, and Pacific saury (Fig. 12). Usage of northern
anchovy in 1994 was below that in 1993. Negative values for market squid and rockfish
indicate these food resources were not caught by auklets as often as they were caught in
trawls in 1994 (Fig. 11, 12).
16
DISCUSSION
Breeding Population Size
The Rhinoceros Auk let population on Aiio Nuevo Island has increased annually since
initial colonization (Fig. 13). Rhinoceros Auklets are suspected to have bred on Ai'io Nuevo
Island as early as 1982, when adults were seeJl carrying fish near the colony (LeValley and
Evens 1982). Burrows were first con finned in 1986 (Lewis and Tyler 1987). Between 1986
and 1989, the number ofburTows increased from 20 to 57, however, between 1989 and 1993
there was a decrease from 57 to 54 bun·ows (Fig. 13 ). These trends are somewhat
misleading, because bun·ows were censused only once per year before 1993. and the number
of open burTows fluctuates throughout a season. In this study, the number of open burrow
entrances varied by as much as 15 within a breeding season.
Comparable annual estimates of the Ai'ro Nuevo population indicate the rate of
population growth may be decreasing. The breeding population increased 51'% f]·om 1993 to
1994 and 12% from 1994 to !995. Occupancy rate values used to derive the population
estimates increased among years (Table 1). Because different methodologies were used to
detennine occupancy, the increase could be biased. When the same occupancy rate was
applied each year, the percent increase in breeding population shifts to 27% from 1993 to
1994 and 8% from 1994 to 1995
From 1993 to 1995, more breeders recruited to nest boxes than natural burrows. After
nest boxes were installed, the number of breeding birds increased from I 02 in 1993 to !54 in
1994; reflecting a 41% increase of breeders in burrows and a 75% increase of breeders in
boxes. The breeding population also increased in 1995, however, the number of breeders in
burTows decreased 3% and the number in boxes increased 52%. To reduce energetic costs,
new recruits and pairs that previously bred in burrows may have breed in nest boxes.
In addition to nest boxes, the population increase could have been facilitated by
increased habitat protection, high juvenile survival, and inunigration. Since 1993, protection
17
and management effm1s (i.e. established restricted areas and designated paths. installed
boardwalks) have reduced the incidental crushing of burrows from 10 in 1993 to one in 1995
(Hester and Sydeman 1996). Juvenile survivorship may be high due to the close proximity to
winter foraging areas (i.e. Monterey Bay and Point Conception; Briggs eta/. 1987. Mason
1997). Inm1igration from Southeast Farallon Island to Ai\o Nuevo Island has been observed
(Hester and Sydeman 1996). Three nest boxes were occupied by breeding adults that were
originally banded on Southeast Farallon Island. Also, an adult was caught in a mist net on
Ai\o Nuevo Island that had been banded only one month earlier on Southeast Farallon Island.
Rhinoceros Auklets tram the Farallon Islands may be a significant source of recruits
for new colonies recently established offCalifomia. Rhinoceros Auklets recolonized the
Fara!lones in 1972, and the population apparently stabilized in the late 1980's (Ainley and
Boekclheide 1990). Since 1986, seven additional breeding areas have been found south of
Atio Nuevo Island (McChesney
r!l
a/. 1995). The number of fledglings produced from Afio
Nuevo Island, assuming high survival and recruitment, could not explain the CtliTent
population growth. At-sea surveys conducted during the breeding season indicate the number
of Rhinoceros Auk lets in waters off central Califomia increased substantially t!·om 1986 to
1992 (Allen 1994 ). This suggests the number of potential recruits from the Fnrallones or
elsewhere may be significant.
Knowledge of population trends, survivorship, and emigration/immigration are needed
to understand population dynamics of Rhinoceros Anklets off California. Annual population
size has not been measured on Southeast Faral!on Island; more than half the habitat is not
accessible. However, capture of Rhinoceros Auk lets with mist nets, conducted since 19R6,
await analysis and could provide inf01mation regarding annual population trends and the
potential of this colony as a source population (Pulliman 1988). Analysis of recapture rates
will provide survivorship estimates for the Farallon Islands, and after additional years of mist
netting, estimates for Atlo Nuevo Island. No infommtion exists on immigration from the large
18
wintering population off Califomia. While infonnation has been gained from long-term
banding efforts. genetic analysis (i.e. microsatallite) may be a more effective means of
investigating emigration/immigration and defining the "metapopulation."
Despite the current growth of the Ai"lo Nuevo Island colony. the breeding habitat may
.
soon be limited and immediate threats have been identified. Due to high winds.. historical
~
habitat alterations, and marine manunals the vegetation on the island is sparse. Reduced
reproductive success of Rhinoceros Anklets was found in areas without vegetation on Teuri
Island, Japan (Miyazaki 1996). The few vegetated patches on Aiio Nuevo Island are
dominated by introduced New Zealand Spinach. Tetragonia tetragonioides. which has a dense
root structure less suitable for buJTowing seabirds. In addition, inter-specific interactions on
Ai"lo Nuevo Island have caused mortality and nest site disturbance. Twenty-one carcasses
were found apparently killed by Peregrine Falcons (Falco peregrinus). Western Gulls have
killed Rhinoceros Anklet fledglings as they pass through gull teJTitories to reach the waters
edge. Two boxes and five buJTows were crushed by elephant seals, 1\limunga angustim.1·tri\
and sea lions, Zalophus ca/i(omianus and Eumetopiasjuballls (Hester and Sydeman 1996).
With additional years of data, the affects of these disturbances on the population may be
modeled.
Reproductive Performance
Rhinoceros Auk let pai1:s in nest boxes initiated breeding earlier each year, and this
difference may have been influenced by improved environmental conditions during the study.
Periods of anomalously wam1-water (El Nii"lo South em Oscillation (ENSO), Nonh Pacific
atmospheric anomalies) result in poor oceanic productivity (Hayward I 993. Lenarz eta/.
1995), and seabirds respond by delaying breeding or abandoning reproductive effm1s.
Rhinoceros Auklets breeding on Southeast Farallon Island showed this response during the
1992-93 ENSO event as the mean egg-laying date was 12 days later than average (PRBO
unpubl. data). Prey availability may have improved from 1993 to 1995 as the marine food
19
web recovered fi·om the wmm-water event of 1992. CTD data indicated average sea surface
temperahires south of the Afio Nuevo upwelling plume were significantly cooler in 1995 than
1993 (Oasis "M2" Buoy 36°N, l22°W: MBARI unpubl. data). This suggests spring
upwelling and resulting food-web productivity may have been more intense near Aiio Nuevo
Island in 1995.
Improved parental quality also may explain the earlier initiation of breeding in 1995.
Although no infonnation exists on the relationship between age and experience and timing of
reproduction in Rhinoceros Anklets, studies of other seabird species indicate experienced pairs
breed earlier than first-year breeders (Saether 1990, Sydeman 1991, 1995, Forslund 1995).
In oceanic regions where prey availability is seasonal (e.g. eastem boundary currents). pairs
must time reproductive efforts such that when chicks hatch, the necessary food resources are
available to feed them (Croxall 197R). Because Rhinoceros Auklets presumably have high site
and mate fidelity. initial recruits in nest boxes at Afio Nuevo Island may be predominately
young breeders. During the first two years after nest box installation, pairs on Aiio Nuevo
Island initiated breeding later than on Southeast Farallon Island, where boxes have been in
place since 1986: theoretically resulting in a more nom1al distribution of age/experienceclasses. In 1995, mean egg-laying dates were similar between colonies. possibly due to
increased experience of pairs breeding in boxes at Aiio Nuevo Island. In the future, when
chicks banded on A1io Nuevo Island retum and recruit to nest boxes, reproductive
performance and chronology of known-age individuals can be monitored to investigate these
relationships.
During the first two years of monitoring nest boxes, productivity was low, with only
33% of the breeding pairs producing independent offspring. Productivity in nest boxes
increased in 1995, but was still less than pairs in nah1ral bunows. The majority of failed
breeding attempts were caused by poor hatching success. Egg-shell thinning caused by
contaminants, nest abandonment, disturbance, low food availability, and other factors can
20
reduce hatching success. Egg-shell samples from At'io Nuevo Island in I 993 had no signs of
thinning (Pyle era/. 1996). Young. inexperienced pairs may be more likely to abandon their
eggs due to under developed socio-sexual behaviors between mated individuals. Hatching
success was significantly greater in burrows. where a higher percentage of sampled pairs
probably had previous breeding
experience~
The effects of disturbance during monitoring was
considered. Evidence from Southeast F arallon Island comparing hatching success of pairs
handled once (nest boxes) and pairs not handled (cave sites) indicated no increased
abandonment (PRBO unpubl. data). Viability of the eggs should be detem1ined (Fry 1996) to
inteq)l'et the causes of low hatching success.
Interestingly, when analyzing nest boxes and burrows together, the population on At'io
Nuevo Island in 1995 (0.70) was 35% more productive than Southeast Farallon Island (0.52:
Fig. 14). The 9-year average productivity on Southeast Faral!on Island is 0.04 (Mclaren cr
ul. 1995: Fig. 14). In undisturbed sites on Protection Island, Washington, 9!'% of
Rhinoceros Auklct pairs produced independent offspring (Wilson 1977), which is the only
area with greater reported productivity than pairs in natural butTows on Ai'Io Nuevo Island.
The miniature surveillance camera was effective in determining productivity in natural
burrows. Because a surprisingly high level of productivity was found in burrows, possible
biases in the method were examined. Thirteen percent of the burrows could not be monitored
due to obstructions, multiple turns and forks, and the length of the burrow. Because
Rhinoceros Auklets likely return to the same site to breed each year, more experienced
breeders may occupy the longer or more complex burrows. For many seabird species.
experienced pairs are more successful at rearing chicks, therefore, the measure of productivity
of pairs in burrows would be under-estimated.
Average growth rates of chicks were greater on At'io Nuevo Island than nearby
Southeast Farallon Island and more northem colonies. Average growth rates varied among
years on Southeast Farallon Island from4.2 to 7.4 g/day (PRBO unpubl. data) and 5.2 to 7.1
21
g/day on Lucy Island, B.C. (Bertram 1988). Chick growth on AI1o Nuevo Island exceeded
these values in both 1994 (8.0 g/day) and 1995 (10.6 g/day). indicating food was readily
available to Rhinoceros Anklets dming chick-rearing.
Relatively high growth rates on A11o Nuevo Island were surprising considering the
high gull density and documented kleptopm:_asitism (pers. obs.). Kleptoparasitism by gulls
decreases growth rates of chicks (Watanuki 1990, Wilson 1993, Harfenist 1996, Miyazaki
1996). Perhaps Rhinoceros Auk lets on Aiio Nuevo Island altered their behavior in response
to this threat. Adult Rhinoceros Auklets anived on the colony after dark: time depending upon
moon phase and cloud cover. On Southeast Farallon Island and other no11hern colonies where
gulls are less dense, adults aJTived at dusk (Scott eta/. 1974, Thoresen 1983, PRBO unpubl.
data ).
Prey Composition and Biomass
Rhinoceros Auklets on Aiio Nuevo Island mostly fed chicks northern anchovies.
Adults fed chicks larger northern anchovies in 1995 than l 993 or 1994. Whether this change
was due to availability or selective foraging is not known. Chicks swallow prey whole and
have difficulty swallowing large fish. Chicks have been found dead with fish protruding from
their mouths upon which they apparently choked (Vermeer 1978). If smaller anchovies are
not locally available when chicks are young, reproductive success may be affected. Chick
growth rates and productivity increased in 1995, therefore, the size classes delivered
( S L= 106-153 mm) were sufficient.
In contrast to other studies, no difference was found in mean bill-load weight among
years or sampling dates (Ven11eer 1980, 1992, Hatch 1982). Nutritional demands of chicks
increase with age (Croxall 1978), requiring parents to deliver more food as chicks grow.
Manipulative experiments have indicated that Rhinoceros Auklet adults adjust provisioning
effm1 in response to chick demand (Be11ram eta/. 1996). Increased bill-loads throughout the
season may not be evident on Aiio Nuevo Island due to: ( 1) large variability in hatching dates
22
within a year (up to 22 days; Table 2), and (2) provisioning efforts may have increased by the
number of meals delivered. which was not measured.
Prey Availability
The impor1ance of prey availability in the determination of seabird population viability
has long been suggested (e.g. Ashmole 19.71, Shuntov 1972). Ashmole ( 1971) concluded
that the location of breeding colonies may be detennined in large par1 by the productivity of
sunounding waters. Prey availability is possibly one factor controlling Rhinoceros Auklet
range expansion in California. By investigating predator-prey relationships of birds at Ario
Nuevo Island, the importance of their feeding ecology to the population's success may be
better understood.
The availability of forage species for Rhinoceros Auklets breeding on A rio Nuevo
Island was investigated using National Marine Fisheries Service trawl data. Although trawl
data were assumed to represent prey abundance and distribution in the environment, numerous
limitations exist with this method. One limitation is the ability ofmidwater trawls to catch lirst
schooling fish. Also, transects can detect large scale patterns but stations are too far apart to
characterize small scale distributions. A prey sampling study should be specifically designed
to elucidate predator-prey relationships. Studies have indicated oceanographic characteristics
play a role in governing seabird distributions (Briggs eta/. 1983, Haney 1987, Croll 1990,
Ainley eta/. 1993, Allen 1994). Physical oceanographic data collected during National
Marine Fisheries Service trawls should be incorporated into further analyses as well.
Another limitation in investigating predator-prey relationships is our knowledge of
Rhinoceros Auklet foraging behavior. Because adult Rhinoceros Auk lets were not commonly
seen can-ying fish at sea during the day and often aiTived completely wet on the colony, adults
were assumed to be diving for chick meals near or after dusk. Simons ( 1981) observed
Rhinoceros Auklets in the Seattle Aquarium foraging in darl01ess. Adults probably forage for
themselves throughout the day, although little is !mown about their ecology when not on
23
colonies. Energetic and physiological requirements of adult birds are different fi·om growing
chicks, and that is often reflected in their diets (Croxa11!987). To fimher understand factors
that may affect Rhinoceros Anklet populations. at-sea surveys near the colonies and trackincr
"'
studies would be beneficial to determine foraging behavior and distributions.
Of the prey species that occurred in ]?oth trawls and chick diets ( co-occuning species),
the species most often delivered to chicks was juvenile and adult northem anchovy. North em
anchovy are small, shori-lived planktivorous fish, typically found in schools.near the surface
(Low 1991 ). At night schools of northern anchovy in the California Cunent occur at an
average depth of 25
111
and do not generally exceed 50
111
(Lenarz 199 I). Northem anchovy
were abundant in PescaderoiDavenport trawls within 4 km of shore. If prey abundance is
sufticient, foraging for chicks just before arTiving at the colony may require less energy and
reduce chances of kleptoparasitism in transport. Atlantic Puffins, Fratercula arcrica, foraged
within 3 to 5 km of Skomcr Island, Wales, and none were seen retuming with tish at
distances greater than 13 km (Corkhill 1973).
Occunence of market squid in trawls and in chick diet was consistent and in low
abundance both years. Although cephalopods rarely dominate alcid chick diets, stomach
content analysis often indicate they are important food for adults (Morejohn eta/. 1978,
Croxall 1987). Stomach analysis of Rhinoceros Auklet adults collected in Monterey Bay.
California, in winter indicated 85% contained market squid (Baltz and Morejohn 1977).
Cephalopods have a lower caloric content than fish (cephalopods= -70 cal/ I OOg, anchovies =
152 cali I OOg; Sidwell 1981 ), therefore, seabirds may select fish to feed their growing chicks.
Species that were abundant in trawls but not fed to chicks may not be available as prey
items. Juvenile Pacific hake, lvfer!ucciusproduc/lls, was caught in three trawls between 14
km and 26 km from Ario Nuevo Island. Large numbers were caught in trawls at 30 m depth,
however, this species generally occurs in waters below 50 m (Lenarz eta!. 1991 ), which is
deeper than Rhinoceros Auklets generally forage (Burger eta/. 1993 ). Juvenile Pacific hake
24
feed on krill and may be available as prey when krill is concentrated at shallow depths. as
evidenced by their occunence in diets of surface feeding shearwaters (Low 1991 ).
Shortbelly rockfish is the most numerous species of rockfish off central Califomia
(Chess !988), and often dominates seabird chick diets on Southeast Farallon Island (Ainley
.
-
and Boekelheide 1990, Sydeman eta/. 1997, PRBO unpubl. data). This species migrates
ve11ically at night to follow their prey. In waters off AI'io Nuevo Island, shortbelly rockfish
were associated with the 200m shelfbreak near Ascension Canyon (Chess 1988). Yoklavich
era/. ( 1996) found larval shm1belly rockfish abundance increased with distance from shore,
and juveniles were most abundant in trawls over Ailo Nuevo canyon. In National Marine
Fisheries Service trawls, most juvenile rockfishes were caught at stations 18 km offshore at
bottom depths of200 m. The few rockfishes that were found in chick diets may have been
taken in these canyons. which are located approximately 9 km southwest of the colony. Both
Pacific hake and shortbelly rockfish may aggregate too far away from A1io Nuevo lsbncl to be
a significant component of chick diets.
Other fish species abundant in trawls but not fed to chicks may be selected against.
When Pescadero/Davenport trawls were combined, juvenile sand dabs (Citlwricluhn spp.)
were fairly evenly distributed in low numbers along the transects. Juvenile sanddabs forage in
surface waters at night (Love 1991 ). They are not known, however, to school in tight
aggregations. If Rhinoceros Auklets rely on vision when foraging, dispersed fish would not
be as visible as schools offish, especially at night. Also, adults may not collect sanddabs for
chicks because the wide shape may be difficult for chicks to swallow whole. Of I ,425 fish
collected from Rhinoceros Auklet chick diets on Southeast Farallon Island between 1987 and
1994, only eight were juvenile flatfishes (PRBO unpubl. data). Larval stage north em
anchovy also were abundant in trawls but this size class (about 25 to 35 mm) was not found in
chick meals. Foraging is energetically expensive and capturing prey below a certain size may
25
not be beneficial. For adults on Afio Nuevo Island, the minimum size may be 45 mm
(standard length), as no fish below this size were found in chick meals.
Species that appeared sporadically, and sometimes abundantly. in chick diets included
Pacific saUJy, juvenile lingcod, and juvenile salmon. During one sampling period in July
1994, chick meals contained more Pacific sa:ny than anchovies. In this study Pacific saUJy
were not caught in trawls. Pacific saUJy migrate to surface waters at night and are most
commonly found 40 to I 00 miles off the coast (Leet eta/. 1992). Optimum temperan1re range
for saury off California is 13.7°C to 17.0°C (Hughes 1974). Off central California, saury may
move inshore in the summer when upwelling relaxes and coastal water temperatures increase.
Lingcod occulTed in chick meals on one sampling date in June 1995. Juvenile lingcod are
pelagic and at about 70 to 80 mm standard length they begin to settle to benthic habitats
(Adams 1993). Rhinoceros Auk lets appear to be utilizing this resource just before or during
settlement. On two sampling dates in 1995, juvenile salmonids were found in chick meals.
Factors a!Tecting the distribution and abundance ofjuvenile salmonids off central California
are complex and range t!·om oceanic fishing interactions to inland habitat degradation. Also,
on Southeast Farallon Island these three species are fed to chicks sporadically (PRBO unpubl.
data). Based on the pattern of occu!Tence of these species in chick meals, it is suspected that
Rhinoceros Auklets may selectively forage for these fish over more locally consistent prey.
However, patchy distributions and low abundance may limit their importance for Rhinoceros
Anklets breeding on Aiio Nuevo Island.
Rhinoceros Auklets on A1io Nuevo Island generally relied on one prey species to rear
chicks and few altemative resources were found near the island. In the absence of northem
anchovy. Rhinoceros Auk lets may be forced to extend foraging trips to utilize juvenile
rockfishes. Allen ( 1994) noted that Rhinoceros Anklets presence at sea was significantly
co!Telated to cooler temperatures during wan11-water years suggesting birds were forced to
forage further from nesting areas. The only other consistently abundant prey species near-
26
shore was market squid. Slower chick growth and reduced reproductive success may be the
result of switching to either prey.
The stan1s of the nonhem anchovy population off central Califomia is not known.
Nmthem anchovy stocks have been exploited in Califomia for reduction and live-bait
fisheries. although central Califomia landings :l!'e low (about I 000 tons per year; Sea Grant
pers. comm.). Anchovy stocks are vulnerable to fishery catches and environmental change as
evidenced by past declines off Baja, Mexico, from over fishing (Hemadezvazquez 1994) and
recent declines otT the Columbia River, probably caused by oceanographic changes (Parrish
pers. comm.). Age and growth studies and tag studies indicated anchovy populations do not
overlap among regions (Baja, southem California Bight, central California, and nol1hern
Califomia Current; PmTish pers. comm.). In addition, southem Califomia Bight landings
were not affected by the crash off Baja. No abundance data or even a direct index exists tor
the central Califomia population (PaJTish pers. comm.). Thus, seabird and marine mammal
diet studies may be the best indicators of anchovy population trends.
Global changes in climatic and oceanographic conditions have been proposed to
explain fluctuations in fish populations in the Califomia Cu1Tent, notably anchovies and
sardines, over the past 1500 years (Baumgmtner eta/. 1995 ). Pacillc sardines are currently
increasing ti·om low levels in the 1970s and biomass levels may have been as high as 62,000
tons in 1990 (Barnes eta/. 1992). Sardines are similar in habitat and behavior to anchovies;
however, due to larger adult size, it is unlikely sardine populations would fill the "anchovyniche" tor seabirds if anchovies declined off central California. Sardines spawn in the south
and young-of-the-year fishes generally occur in central California in large numbers only
during wan11-water years (Lluchbelda eta/. 1991 ). In recent years, juvenile Pacific sardines
have occmTed in low percentages in Rl1inoceros Auklet and Conunon Mum; chick diets off
central Califomia (PRBO unpubl. data). Additional years of monitoring will detem1ine if
Pacillc sardine increases in Rl1inoceros Auklet chick diets on Aiio Nuevo Island.
27
Due to their reliance on one food resource, R11inoceros Auk lets on Afio Nuevo Island,
as well as other near-shore colonies, may be considerably affected during wan11-water events.
Comparisons have been made between the Pem and Califomia Cmrent systems stating that
seabird populations in the California Cunent are less affected by warm-water events due to a
more diverse prey choice (Ainley era!. 1995). In the Pem Cunent, birds rely on just one
species, the anchoveta (Engraulis ringens), and mass bird mortalities have been observed
during ENSO periods (Mmvhy 1981 ). Seabird colonies in Califomia that lie near the
continental shelf are sunounded by heterogeneous habitats that provide breeding birds a
diversity of prey (i.e. Channel Islands. Farallon Islands; Ainley and Boekelheide 1990, Hester
and Sydeman 1996 ). However. numerous colonies are near the mainland and breeding birds
are restricted to a more homogenous environment while rearing chicks. In addition to
Rhinoceros Auk lets on Afio Nuevo Island, Brown Pelican (Pelecanus occidenra!is
calijiJmicus) colonies are dependent on one species, the northern anchovy (Anderson 1982).
Few diet studies have been conducted on other nearshore predators (e.g. harbor seals; Harvey
e/ a!. 1995, Oxman 1995, Trumble 1995), but additional species (i.e. Common Murre, Pigeon
Guillemot, cormorants, juvenile pinnipeds) may also rely on a single food resource. During
the 1992 ENSO, northem anchovy landings in Monterey Bay, California, were less than onequm1er of normal levels, equaling a I 0-year low (Bob Leos pers. comm). In the event
environmental anomalies alter northern anchovy populations, seabirds breeding near-shore
may be more drastically atTected than on offshore colonies.
The comparative analysis of Rhinoceros Auklet chick diet and trawl catches
demonstrated that: I) adults in general are not feeding chicks the most abundant resources in
their foraging area; 2) adults may be foraging selectively or abundant fish species are not
available as prey; 3) adults appear to be foraging for their chicks near the colony; and 4) chick
provisioning may be significantly affected if not1hem anchovy stocks decline.
28
SUMMARY AND CONCLUSIONS
The Rhinoceros Auklet breeding population on Aiio Nuevo Island increased 69%
from 1993 to 1995. Due to habitat degradation and other threats, the population may soon
be limited. Methods tested (i.e. bu1Tow censusing, miniature camera methods) in this
study to accurately detem1ine occupancy rat:s could be applied to other colonies to
investigate variability within and among populations. This infmmation will aid in
documenting annual population size fluctuations.
Numerous factors regulate seabird populations. In this study, reproductive
perfonmnce and prey utilization were investigated. Productivity of the population was
relatively high (0.30 to 0.83 chicks per breeding pair). Nest boxes were successful at
providing protected breeding sites. but reproductive performance of pairs breeding in nest
boxes was less than those in natural burrows. As the breeding population in nest boxes
approaches the age/experience-class distribution in buiTows, productivity should continue
to increase. Because Rhinoceros Auklets are capable of fledging chicks in years of poor
oceanic conditions, monitoring more sensitive variables than productivity may be valuable.
Immediate responses to anomalies may be detected by fluctuations in timing of breeding
and chick growth rates. Foraging conditions in 1995 were apparently more favorable for
breeding Rhinoceros Auklets, as pairs initiated breeding earlier and chicks grew faster.
Food resources were suflicient and not limiting the population based on concurrent
midwater trawls for available prey. However, no large-scale oceanic anomalies (i.e.
ENSO, North Pacific pressure anomalies) occllJTed during this study. Rhinoceros Auk lets
on Ai\o Nuevo Island rely primarily on northem anchovy stocks during the summer to rear
young. This is further evidence of the importance of this stock in the central Califomia
coastal ecosystem.
29
Knowledge of Rhinoceros Auklet ecology in central Califomia will guide
.....
'-P
......
management and protection programs. Due to the size and accessibility of the Afio Nuevo
colony. it is an ideal place to investigate factors regulating population growth. This stltdy
could not address age-related effects. although indirect evidence suggests they may be
substantial. Future research could focus on)mown-age individuals when cohmts from
Aiio Nuevo Island return to breed. The Rhinoceros Auklet is the only alcid species in
Califomia that has continued to increase in number during the past two decades (Sowls et
a!. 1980. Carteret a!. 1992). To understand this process, knowledge of recruitment and
survivorship is needed. Also. in the event of mass mottality (i.e. oil spills). such
infom1ation is needed so restoration effmts can be applied properly.
30
LITERATURE CITED
Adams, P.B. (ed.). 1992. Progress in rockfish recruitment studies. Southwest Fisheries
Science Center Administrative Rep011 T-92-0 I.
Adams, P.B., S. Ralston, and I.E. Laidig. 1993. Occurrence of an exceptional catch of
pelagic juvenile lingcod (Ophiodon e/ongarus) off Point Reyes. California. Fish.
Oceanogr. 2:97-100.
Ainley, D.G., A.R. DeGange, L.L. Jones, and R.J. Beach. 1981. Monality of seabirds in
high seas salmon gill net. Fisheries Bulletin 79:800-806.
Ainley, D.G. and R.J. Boekelheide (eds.). 1990. Seabirds of the Farallon Islands.
Stanford University Press, Palo Alto, CA.
Ainley, D.G .. W.J. Sydeman, R.H. Parrish, and W.H. Lenarz. !993. Oceanic factors
influencing distribution of young rockfish (Sebastes) in central Califomia: A
predator's perspective. Ca!COFI Rep. 34.
Ainley, D.G., R. Podolsky, L. de Forest, G. Spencer, and N. Nur. 1995. The ecology of
Newell's Shearwater and Dark-rumped Petrel on the island of Kauai. Final Report
Task 2 to Electrical Power Research Institute, Palo Alto, CA.
Ainley. D.G., R.L Veit. S.G. Allen, L. Spear, and P. Pyle. Variations in marine bird
communities of the California CuiTent, 1986-1994. Ca!COFI Rep. 36:72-77.
Allen, S.G. 1994. The distribution and abundance of marine birds and mammals in the Gulf
of the Farallones and adjacent waters, 1985-1992. PhD Dissertation, University of
Califomia, Berkeley.
Anderson, D. W., F. Gress, and K.F. Mais. 1982. Brown Pelicans: Influence of food
supply on reproduction. Oikos 39:23-31.
Ashmole, N.P. 1971. Sea bird ecology and the marine environment. Pp. 223-286. In D.S.
Farner and .J.R. King (eds.), Avian Biology I. Academic Press, New York
Bailey, E., and G.W. Kaiser. 1993. Impacts of introduced predators on nesting seabirds in
the northeast Pacific. Pp. 218-226. In Vermeer, K., K.T. Briggs, K.H. Morgan,
and D. Siegel-Causey (eds.), The Status, Ecology, and Conservation of Marine Birds
of the Nm1h Pacific. Canadian Wildlife Service, Ottawa, Ontario.
Baltz, D.M. and G.V. Morejohn. 1977. Food habits and niche overlap of seabirds
wintering on Monterey Bay, California. Auk 94:526-543.
Bames J. T., L.D. Jacobson, A. D. MacCall, and P. Wolf. 1992. Recent population trends
in abundance estimates for the Pacific sardine (Sardinops sagax). Ca!COFI Rep. 33.
Baumgartner, T.R., A. Soutar, and W. Riedel. 1996. Natural time scales of variability in
coastal pelagic fish poplulations of the Califomia CuiTent overthe past 1500 years:
Response to global climate change and biological interaction. Califomia Sea Grant,
Report of Completed Projects, 1992-1995. LaJolla, CA. pp. 31-37.
31
Bel1ram. D.F. 1988. The provisioning of nestlings by parent Rhinoceros Anklets,
Cerorhinca monocerata. Masters Thesis, Simon Fraser Univ., Burnaby, BC.
Bertram. D.F. and G.W. Kaiser. 1993. Rhinoceros Anklet ( Cerorhinca monocerata)
nestling diet may gauge Pacific sandlance (Ammodytes hexapterus) recruitment. Can.
J. Fish. Aquat. Sci. 50:1908-1915.
Bei1ram, D.F., C.V.J. Welham, and R.C. Ydenberg. 1996. Flexible effort in breeding
seabirds according to nestling age ancl mass~ Can. J. Zoo!. 74:1876-1881.
Briggs, K.T., W.B. Tyler, D. B. Lewis, and K.F. Dettman. 1983. Seabirds of central and
northem Califomia, 1980-1983: status, abundance. and distribution. Pub!. Center for
Coastal Marine Studies, University ofCalifomia, Santa Cruz.
Briggs, !CT., W.B. Tyler. D.B. Lewis, and D.R. Carlson. 1987. Bird Communities At
Sea OffCalifomia: 1975 to 1983. Studies in Avian Biology 11.
Burger, A. E., R.P Wilson, D. Garnier, and M.P.T. Wilson. 1993. Diving depths, diet,
and underwater foraging of Rhinoceros Anklets in British Columbia. Can. J. Zoo!.
71 :2528-2540.
Carter, H.R., G.J. McChesney, D.L. .Jaques. C.S. Strong, M.W. Parker. J.E. Takekawa,
D.L. .Tory, and D.L. Whitworth. 1992. Breeding populations of seabirds in
California, 1989- 1991. Unpublished Report of the U.S. Fish and Wildlife Service.
Dixon, CA.
Chess, .J.R., S.E. Smith and P.C. Fischer. 1988. Trophic relationships of the shortbelly
rockfish, Sebastesjordani, off central Califomia. CalCOFl Rep. 29.
Clark, F.N. 1928. The weight-length relationship of the Califomia Sardine (Sardina
caeru/ea) at San Pedro. Fish Bulletin 12.
Cody, M.L. 1973. Coexistence, coevolution, and convergent evolution in seabird
communities. Ecology 54: 31-44.
Croll, D.A. 1990. Physical and biological determinants of the abundance, distribution, and
diet of the Common Mune in Monterey Bay, California. Studies in Avian Biology
14:139-148.
Croxall, J.P. (ed.). 1987. Seabirds, Feeding Ecology and Role in Marine Ecosystems.
Cambridge Univ. Press. 408 pp.
Decker, M.B., G.L. Hunt, G.V. Byrd, and R.J. Beamish (eds). 1995. The relationships
among sea-surface temperature, the abundance of juvenile walleye pollock (Theragra
chalcogramma ), and the reproductive perfonnance and diets of seabirds at the Pribilof
Islands, southeastem Bering Sea. Pp. 425-437. In Climate Change and Northem
Fish Populations. Can. Spec. Pub!. Fish. Aquat. Sci. 121.
DeGange, A.R. and R.H. Day. 1991. Mm1ality of seabirds in the Japanese land-based
gillnet fishe1y for salmon. The Condor 93:251-258.
32
Dyer. P.K. and G.J.E. Hill. 1991. Solution to the problem of detem1ining the occupancy
stan1s of Wedge-tailed Shearwater, Puffin us pacificus, bunows. Emu 91:20-25.
Ford. R.G., J.L. Casey. C.H. Hewitt, D.B. Lewis, D.H. Vaaroujean, D.R. Warrick. and
W.A. Williams. 1991. Seabird monality resulting from the Nestucca oil spill
incident. winter 1988-89. Report for Washington Dept. Wildlife. Ecology Consulting
Inc., Portland. OR.
Forslund, P. and T. Part. 1995. Age and reproduction in birds--tests and hypotheses.
TREE 10:374-378.
Fry, D.M. and J.A. Davis. 1996. A video candling device for evaluating egg viability of
birds in the field. Abstract, Pacific Seabirds 23:33.
Grinnell. J. 1926. The evidence to the fom1er breeding of the Rhinoceros Auklet in
California. The Condor 28:37-40.
~
Gruber. F. 1884. Die Seevogel der Farallone-Inseln. Zeitschr. f. d. gesammte Orn .. I,
Heft 2:167-172.
Hamer, !(.C., P. Monaghan, J.D. Uttley, P. Walton, and M.D. Burns. 1993. The
inlluence of food supply on the breeding ecology of Kittiwakes Risso tridacn'ia in
Shetland. Ibis 135:255-263.
Haney, .!.C. 1987. Ocean intemal waves as sources of small-scale patchiness in seabird
distributions on the Blake Plateau. Auk 104: 129-133.
Harfenist, A. and R. Y den berg. 1995. Effects of growth-rate variation on Hedging of
Rhinoceros Auklets ( Cerorhi11ca monocerata ). The Auk 112:1-17.
Harvey, J.T., R.C. Helm, and G.V. Morejohn. 1995. Food habits of harbor seals
inhabiting Elkhom Slough, Califomia. California Fish and Game 81-1:1-9.
Hatch, S.A. 1982. Nestling diet and feeding rates of Rhinoceros Auklets in Alaska. In
D.N. Nettleship, G.A. Sanger, and P.F. Springer (eds.), Marine Birds: Their
Feeding Ecology and Commercial Fisheries Relationships. Canadian Wildlife Service,
Proceedings of the Pacific Seabird Group Symposium.
Hayward, T.L. 1993. Preliminary observations of the 1991-1992 E! Nino in the California
Cunent. CalCOFI Invest. Report 34:21-29.
Heermann, A.L. !859. Pac. R.R. Rep.] 0. part 4, 2:75.
Hemandezvazquez, S. 1994. Distribution of eggs and larvae from sardine and anchovy off
Califomia and Baja Califomia, 1951-1989. CalCOFI Rep. 35:94-107.
Hester, M.M. and W.J. Sydeman. 1993. Breeding biology, food habits, and conservation
ofR.hinnoceros Anklets, Cerorhinca monocerata, on Aiio Nuevo Island, California.
Report to the State of California Dept. of Parks and Recreation, Bay Area District.
Hodder, J. and Graybill, M. 1985. Reproduction and survival of seabirds in Oregon during
the 1982-1983 El Nil'io. The Condor 87:535-541.
33
Hudson .. G.E., K.M. Hoff, J.V. Be:·ge, and E.C. Trivette. 1969. A numerical study of the
wmg and leg muscles ofLan and Alcae. Ibis 111:459-524
Hughes. S.E. 1974. Stock composition, growth, mm1ality. and availability of Pacific
saury. Cololabis sa ira, of the northeastem Pacific Ocean. Fishery Bulletin 72( 1): 121131.
lvlev, V.S. 1961. Experimental Ecology of the Feeding of Fishes. Yale University Press.
New Haven, Conn.
Leet. W.S .. C.M. Dewees. and C. W. Haugen. 1992. Califomia's Living Marine
Resources and their Utilization. Sei\ Grant Extension Publication UCSGEP-92-12.
Lenarz, W.H., R. Larson. and S. Ralston. 1991. Depth distributions of late larvae and
pelagic juveniles of some fishes of the Califomia Cunent. CalCOFI Rep. 32.
Lenarz. W.H., D.A. Vantresca, W.M. Graham. F.B. Schwing. and F. Chavez. 1995.
Explorations ofEl Nino events and associated biological population dynamics off
central California. CalCOFI Rep. 36:106-119.
Leschner. L.L. 1976. The breeding biology of the Rhinoceros Auk let on Destruction
Island. Masters Thesis, University of Washington, Seattle.
LeValley. R. and J. Evens. 1982. The nesting season: Middle Pacitic coast region.
American Birds 36: 1011-1015.
Lewis, D. B. and B. Tyler. 1987. Management Recommendations for Coastal Terrace and
Island Resources at Afio Nuevo State Reserve. Unpublished Report to California
Dept. of Parks and Recreation, Afio Nuevo State Reserve. Institute of Marine
Sciences, University of California, Santa Cruz.
Lluchbelda, D., D.B. Lluchcota, S. Hernandezvasquez, and C.A. Salinaszavala. 1991.
Sardine and anchovy spawning as related to temperature and upwelling in the
Califomia CtnTent System. CalCOFI Rep. 32:105-111.
Love. M. 1991. Probably More Than You Want To Know About the Fishes of the Pacific
Coast. Really Big Press, Santa Barbara, CA. 213 pp.
Low, L. (ed.). 1991. Status of Living Marine Resources off the Pacific Coast of the United
States as Assessed in 1991. U.S. Dept. Comm .. NOAA Tech. Memo. NMFS
F/NWC-210. 69 pp.
Martin, A.R. 1989. The diet of Atlantic Puffin, Fratercula arcrica, and Northem Gannet,
Sui a bassana, chicks at a Shetland colony during a period of changing prey
availability. Bird Study 36:170-180.
Mason. J. 1997. Distribution and abundance of seabirds in Monterey Bay, Califomia.
Masters Thesis, Moss Landing Marine Laboratories, CA.
34
McChesney, GJ., H.R. Cm1er •. and D.L. Whitwm1h. 1995. Reoccupation and extension
of southem breedmg lnmts of Tufted Puffins and Rhinoceros Anklets in Califomia
Colonial Waterbirds 18:79-90.
'·
McLaren, E.B .. WJ. Sydeman, and P. Pyle. 1992. Southeast Faral!on Island Annual
Seabird Repon to U.S. Fish and Wildlife Service.
McLaren. E. B., T.G. Schuster, W.J. Sydeman and P. Pyle. 1995. Population Size and
Reproductive Success of Seabirds on Southeast Farallon Island, 1995. Repm1 to the
U.S. Fish and Wildlife Service.
Messersmith, J.D. 1969. The Northern anchovy (Engraulis morda-r) and its fishery, 19651968. Fish Bulletin 147.
Miller. D.J. and R.N. Lea. 1972. Guide to the Coastal Marine Fishes of Califomia. Fish
Bulletin 157.
Miyazaki, M. 1996. Vegetation cover, kleptoparasitism by diurnal gulls. and timing of
an·ival of nocturnal Rhinoceros Anklets. The Auk 113:698-702.
Morejohn, G.V., J.T. Harvey. and L.T. Krasnow. 1978. The importance ofLoligo
opal esc ens in the food web of marine vertebrates in Monterey Bay. Califomia. Fish.
Bull. 169:67-97.
MuqJhy. R.C. 1981. The guano and anchovetta fishery. Pp. 81-106 In MH. Glantz and
J.D. Thompson (eds.), Resource Management and Environmental Uncertainty:
Lessons from Coastal Upwelling Fisheries. Wiley, New York.
Oxman. D.S. 1995. Seasonal abundance, movements, and food habits of harbor seals
(Phoca l'itulina richard,i) in Elkhorn Slough, California. Masters Thesis. Moss
Landing Marine Laboratories, CA.
Page, G.W .. H.R. Carter, and R.G. Ford. 1990. Numbers of seabirds killed or debilitated
in the 1986 Apex Houston oil spill in central Califomia. In S.G. Sealy (ed.). Auks At
Sea. Studies in Avian Biology 14.
Paine, R.T., J.T. Wooton, and P.D. Boersma. 1990. Direct and indirect effects of
Peregrine Falcon predation on seabird abundance. The Auk 107: l-9.
Pan·ish. R.H., D. Mallicoate, and K.F. Mais. 1983. Northern Anchovy: Fishery
Management Plan. Final Supplementarey EIS.
Piatt, J.F., S.A. Hatch, B.D. Roberts, W.W. Lidster, J.L. Wells, and J.C. Haney. 1988.
Populations, productivity, and feeding habits of seabirds on St. Lawrence Island,
Alaska. Final Report to Min. Manage. Serv. OCS Study MMS 88-0022. Anchorage,
AK.
Pulliman, H.R. 1988. Sources, sinks, and population regulation. The American Naturalist
132:652-651.
35
Pyle. P., WJ. Sydeman. and E.B. McLare~1. 1995. Egg shell thickness and hatchability as
related to organochlonne concentratwns m seabtrds of the Gulf of the Farallones.
Report to the Gulf of the Farallones National Marine Sancnmry.
Richardson, F. 1961. Breeding biology of the Rhinoceros Auklet on Protection Island.
Washington. The Condor 63:456-473.
Rosenberg, D., J. Gervais, M. Fry, and L. Tn.!lio. 1997. Evaluation of contaminant
residues in Bun·owing Owl populations. Unpubl. progress report.
Rosenfeld, L.K., F. B. Schwing, N. Garfield, and D. E. Tracy. 1994. Bifurcated flow from
an upwelling center: a cold water source for Monterey Bay. Continental Shelf Res.
14:931-964.
Saether, B. 1990. Age-specific variation in reproductive performance of birds. Pp. 251281. In D.M. Power (ed.). Current Ornithology 7. Plenum Press. New York.
Scott. M.J., W. Hoffman, D.G. Ainley, and C.F. Zeillemaker. 1974. Range expansion and
activity pattems in Rhinoceros Auklets. Westem Birds 5:13-20.
Sidwell, V.D. 1981. Chemical and nutritional composition of finfishes, whales.
crustaceans. mollusks, and their products. NOAA Tech. Mem. NMFS F/SEC-11.
432 pp.
Shuntov, V.P. 1972. Sea birds and the biological structure of the biological structure of the
ocean. (Trans. tl·om Russian). Bur. S-port Fish. Wild!. and Nat( Sci. Found., Wash.
D.C. Nat. Tech. lnf. Serv. TT 74-55032.
Simons, T.R. 1981. Feeding behavior of captive alcids. U.S. Dept. Int. Nat. Park Serv.
Maui, Hawaii. Unpubl. Rep.
Sowls, A.L., A.R. DeGange, J. W. Nelson, and G.S. Lester. 1980. Catalog of California
Seabird Colonies. U.S. Department of Interior, U.S. Fish and Wildlife Service,
Biological Services Program. FWS/OBS 37/80. 371 pp.
Spratt, J.D. 1981. Status ofthe Pacific herring, Clupea harenguspallasii, resource in
California 1972 to 1980. Fish Bulletin 171.
Storer, R.W. 1945. Stuctural modifications in the hind limb of the Alcidae. Ibis 87:433456.
Strauch, J.G., Jr. 1985. The phylogeny of the Alcidae. The Auk 102:520-539.
Summers, K. R. and R. H. Drent. 1979. Breeding biology and twinning experiments of
Rhinoceros Auk lets on Cleland Island, British Columbia. The Munelet 60:16-22.
Sydeman, W.J., K.A. Hobson, P. Pyle, and E.B. McLaren. 1997. Trophic relationships
among seabirds in central California: combined stable isotope and conventional dietary
approach. The Condor 99:327-336.
36
I
!
Sydeman, WJ., J.F. Penniman, T.M. Penniman, P. Pyle, and D.G. Ainley. 1991.
Reproductive perfonnance in Westem Gulls: Effects of parental age, timing of
breedmg and year 111 relal!on to food availability. J. Anim. Ecol. 60:135-149.
Sydeman. W.J .. P. Pyle. S.D. Emslie, and E.B. McLaren. 1995. Causes and
consequences oflong-tem1 partnerships in Cassin's Auklet. In J. Black and B. Ens
(eds.), Partnerships in Birds: The Ecology ofMonogomy. Oxford Univ. Press,
Oxford.
Thoresen, A.C. 1983. Diurnal activity and·social displays of Rhinoceros Auklets on Teuri
Island, Japan. The Condor 85:373-375
Trumble, S.J. 1995. Abundance, movements, dive behavior, food habits, and mother-pup
interactions of harbor seals (Phoca vitulina richardsi) near Monterey Bay, California.
Masters Thesis. Moss Landing Marine Laboratories, CA.
Vermeer, K. 1978. Extensive reproductive failure of Rhinoceros Auklets and Tufted
Puffins. Ibis 120: 112.
Vermeer, K. and L. Cullen. 1979. Growth of Rhinoceros Auklets and Tufted Puffins.
Triangle Island, British Columbia. Ardea 67:22-27.
Vermeer, K. 1980. The importance of timing and type of prey to reproductive success of
Rhinoceros Auklets, Ccrorhinca monocerata. Ibis 122:343-350.
Vermeer, K. and S.J. Westrheim. 1982. Fish changes in diets of nestling Rhinoceros
Auklets and their implications. In D.N. Nettleship, G.A. Sanger, and P.F. Springer
(eds.), Marine Birds: Their Feeding Ecology and Commercial Fisheries Relationships.
Canadian Wildlife Service, Proceedings of the Pacific Seabird Group Symposium.
Vermeer, K. and K. Devito. 1986. Size, caloric content, and association of prey fishes in
meals of nestling Rhinoceros Anklets. The Murrelet 67:1-9.
Watannki, Y. 1990. Daily activity pattern of Rhinoceros Anklet and kleptoparasitism by
Black-tailed Gulls. Om is. Scand. 21 :28-36.
Wilson, U.W. 1977. A study of the biology of the Rhinoceros Auklet on Protection Island,
Washington. Masters Thesis, University of Washington, Seattle.
Wilson, U.W. 1986. Artificial Rhinoceros Auklet burrows: A useful tool for management
and research. J. Field Ornithol. 57(4):295-299.
Wilson, U.W. and D.A. Manuwal. 1986. Breeding biology of the Rhinoceros Anklet in
Washington. The Condor 88:143-155.
Wilson, U.W. \993. Rhinoceros Auklet burrow use, breeding success, and chick growth
in gull-free vs gull-occupied habitat. J. Field Ornithol. 64:256-261.
Yoklavich, M., V.J. Loeb, M. Nishimoto, and B. Daly. 1996. Nearshore assemblages of
larval rockfishes and their physical environment off central Califomia during an
extended El Nino event, 1991-1993. Fishery Bulletin94:766-782.
37
Table l. Estimates of Rhinoceros Auklet breeding population on Afio Nuevo Island in areas l-5, 1993 to 1995.
Number of natural burrows t·cJlects the number consistently open ti·om mid April through June. Occupancy
rates represent the propo11ion of bwTows occupied by breeding pairs.
w
Year
Number
Boxes
Installed
Number
Breeding Pairs
in Boxes
Number Natural
BuiTows
1993
40
12
54
Occupancy
Rate
Estimated
Breeding Pairs
in Burrows
Total Breeding
Population
boxes + burrows
0.73
39
51 pairs
'(l02birds)
00
1994
56
21
69
0.81
56
77 pairs
(!54 birds)
1995
56
'7
.)_
64
0.85
54
86 pairs
(172 birds)
Table 2. Mean, standard error (SE), and range of days for egg laying, hatching, and Jledging. Also, date that
adults were first observed attending nest sites. In 1995, data are given for nest boxes and burrows.
Year
w
'()
Adults tlrst found
attending nest sites
during the day
Egg-Laying
Hatching
Fled ainu
b
b
1993 Boxes
1994 Boxes
1995 Boxes
1995 Burrows
mean± SE (n)
(range)
mean± SE (n)
(range)
mean± SE (n)
mean± SE (n)
(range)
(range)
27 April
11 April
18 March
22 April
16 May± 3.5 (12)
12 May± 2.8 (25)
8 May± 2.8 (32)
I May± 2.3 (12)
(27 April-9 June)
(24 April-4 June)
( 17 April-IS July)
(22 April-16 May)
23 .June± 2.7 (5)
11 June± !.8 (8)
14 June± 1.5 ( 16)
18 June± 2.8 (10)
( 16 June-4 .July)
(4 June-IS June)
(J I Mny-5 .July)
(4 June-26 .June)
15 Aug.± 2.5 (4)
31 July± 3.0 (7)
29 .July± 2.5 ( 16)
30 July± 2.0 (9)
(8 Aug-IS Aug)
I IS July-S Aug. l
I 10 July-20 Aug.)
(21 .July-S Aug.)
Table 3. Mean (grams/day) and standard en·or (SE) for growth rates of Rhinoceros
Auklet chicks (during the linear growth phase) reared in nest boxes on Ano Nuevo
Island. 1993 to 1995.
Year
Growth Rate
SE
11
0.7
4
0.5
7
0.6
16
mean (g/day)
1993
7.0
age 14-42
1994
8.0
age I 0-40
1995
10.6
age 10-40
40
Table 4. Reproductive perfonnance (hatching success. fledging success, productivity) of
Rhinoceros Auklet pairs breeding in nest boxes from 1993 to 1995 and burrows in 1995 on
Aiio Nuevo Island. Values are proportions.
Nest Site
Type
1993
Year
(pairs)
Hatching
Success
Fledging
Success
Productivity
(fledglings/pair)
nest boxes
12
0.42 (5/12)
0.80 (4/5)
0.33 (4/12)
1994
nest boxes
21
0.38 (8/21)
0.88 (7/8)
0.33 (7/21)
1995
nest boxes
32
0.50 ( 16/32)
1.0 (16/16)
0.50 ( 16/32)
1995
burrows
47
0.83 (39/47)
1.0 (39/39)
0.83 (39/47)
n
-""
.
Table 5. Mean weight (grams) and standard enor (SE) of bill-loads delivered to
Rhinoceros Auklet chicks by standardized sample date (sample date minus mean
hatching date each year, 1993-1995 pooled) and by year, 1993 to 1995.
Standardized
Sample Date (days)
Mean Bill-load (g)
SE
n
10-20
28.1
2.1
:w
21-30
27.1
5.2
14
31-40
27.7
2.6
20
41-50
28.2
5.4
6
Year
Mean Bill-load (g)
(range)
SE
11
1993
26.0
(5.9- 49.6)
2.6
19
1994
28.3
3.3
24
2.3
17
(2.3- 75.7)
29.0
1995
(12.4- 40.2)
42
Table 6. Occurrence (proportions) of prey species in Rl1inoceros Auklet chick meals
(diet) and Davenport/Pescadero trawls (trawl) in 1993 and 1994. Co-occun·ing species
are those caught in trawls and fed to chicks. "Other taxa" from trawls comprised
13 species in 1993 and 15 species in 1994. For diet data, meals= number ofbil!loads
and n =total number of fish. Trawl data collected by National Marine Fisheries Service.
1993
Co-occuning Species
Engraulis mordax. ad. &juv.
1994
1994
Diet
Trawl
Diet
Trawl
mea\s::=20
n=84
trawls=l3
n=42,239
mea\s::::25
n=99
trawls=9
0.82
0.02
0.69
0.1
0.05
0.08
0.12
0.03
0.03
0.05
Loligo opalescens
Sebastesjorcla11 i
1993
0.06
Sebastcs spp.
0.01
n=l.542
. 0.06
Non Co-occurring Species
Ammoc(\'le.1· he:raplerus
0.01
Bathylagidae
0.02
0.08
Citharichthys spp.
0.02
0.2
Clupea pallasi
0.06
0.16
Co/o/abis .mira
Engraulis mordax, larval
0.01
0.27
Merluccius productus
0.68
0.02
Myctophidae
0.11
0.06
Ophiodon elongatus
0.04
0.02
Pleuronectidae
Sal'(/inops sagax
0.03
0.06
other taxa < I%
0.01
43
0.02
t\NO NUEVO ISLAND
J71lfJ' JW N. ! 22 20' IJ'J'' W
"
'"'
~~
'CAll- J:f 11-1 T
LEGEND
c:::::J
!WAll Urccdmg
Arl!;J~
( 1-h)
A -Sidler Blind
1J - Oulhousc
C- lllockhou~c
D- Foghuu~c
E- (\~lcnl
F -1-tght T"'l'l'l
(i- Hlind ILl
D
0
II - Blind 1117
I - l.ig/11 Kccp~·r's
I lome
Figure I. Aiio Nuevo Island showing Rhinoceros Auklet breeding areas ( 1-6), boardwalks, and island structures.
37.9
-<!r
119
MWT
Rhinoceros Auk.let Colony
~
z
~
37.4
<l.)
Pescadero Transect
"0
-~
¢-
¢-
¢-¢- ¢-
134
1!33
132. 131 130
~
"'
ANI C1
...l
36.9
36.4
124.0
Davenport Transect
<>126
«>-<>
125124 123 122
123.0
122.0
Longitude ( "W )
Figure 2. Map showing National Marine Fisheries Service (NMFS) trawl stations used in
this study. Open circles represent trawl stations (MWT) and the closed circle represents
Afio Nuevo Island (ANI). Map modified from NMFS.
45
10
8
1993
6
4
2
0
10
"'
00
g
1994
00
~
"'-<
0
....
<U
4
0
..0
a
z
:::l
2
0
10-
8-
1995
6-
42I)
I
f--- April
R
May
June
Figure 3. Distribution of Rhinoceros Auklet egg laying dates on Aiio Nuevo Island,
1993-1995. Hatched bars represent eggs that did not successfully hatch. One egg
(from a re-lay attempt) successfully hatched on 31 May 1995.
-16
% OCCURRENCE
n
= 20
51
17
%BIOMASS
12
n =' 20
51
17
12
100
90
80
70
[]]]
Sard!Herr
60
~
Herring
&1l
II
fll
Sardine
50
40
Rockfish
Anchovy
30
20
10
0
10-Jul
24-lul
31-Jul
7-Aug
10-Jul
24-Ju1
31-lul
7-Aug
Netting Date
Figure 4. Percent occurrence and percent biomass of prey species in Rhinoceros Auklet
chick diet for each netting date in 1993. Numbers above columns are sample sizes of
prey items.
47
%
OCCURRENCE
n=7
55
17
%
n· =7
20
BIOMASS
55
17
20
100
90
90
80
80
70
70
60
i:i
8"'
"""'
60
50
50
40
40
30
30
20
20
10
10
a
EJ
Saury
~
Sandlance
~
San.line
0
Squid
•
lA
Rockfish
Anchovy
0
24-Jun
4-Ju1
15-Ju1 23-Jul
24-Jun
4-Ju1
15-Ju1 23-Ju1
Netting Date
Figure 5. Percent occurrence and percent biomass of prey species in Rhinoceros Auklet
chick diet for each netting date in 1994. Numbers above columns are sample sizes of
prey items.
48
Figure 6. Percent occurrence and percent biomass of prey species in Rhinoceros Auklet
chick diet for each netting date in 1995. Numbers above columns are sample sizes of
prey items.
49
160
140
120
~
8
....
8
100
3
c::
'--'
..c
~
•
80
OJJ
c::
OJ
-' 60
I
I
I
ElI
B
9
16
8
Ef3
5
88
2
40
!56
3
8
..c
"'='
20
0
Q
:::l
0..
0..
c:l
'J)
u
0
....
Ul
<.;:;
u
c:l
0..
c
c::
"'
Cll
11.)
....u
3
"'c::='
,.,
'J)
"J
·-u
~
"'
0..
>-,
>
Q
..c
§
c::
'-
'-'
-:5
3
"'0='
u
OJJ
....
'-'
c::
OJJ
~
''-<
..
'J)
--'
u
:..;:::::
,.,u
Cl..
z
c::
~
u
0..
0..
'J)
..c
'J)
,-
"""'u
:...:::;
~
:d
~
::::::
~
"'
<.;:;
..:.::
u
Q
'>-,
'!)
.0
::l
:;"
'J)
~
OJ
~
2
....
0
..c
~
Cll
Figure 7. Mean length (horizontal line), range (vertical line), and standard error
(box represents± one SE) of each prey species in Rhinoceros Auklet chick diets
from 1993 to 1995. Values are standard lengths for all species except Pacific
herring which only fork length was recorded. Values for market squid are mantle
lengths. Numbers below bars are sample sizes.
50
Ill
Engraulis 1993
I
[] Engraulis 1994
450
-
depth (m)
I
600
-
400
~
- 500
350
I
"'
... 300
0..
"'
-;;
.g
·;;
250
200
:;:; 150
.5
"" 100
·-~
L
I-~
~
~
P..
1- 300 .g
E
0
200
50
0
4ooE'
""'
I
I
I
0)
p-
IIIII-
v
r-'
r- 100
llllll
E E
Davenport
::g
..D
E
E
0
Pescadero
Figure 8. Distribution and abundance of northern anchovy, Engraulis mordax, Uuvenile
and adult stages) in trawls along Davenport and Pescadero transects. Bars represent
abundance and lines represent bottom depth (m). Numbers on x-axis are distance from
shore (km) of the trawl station. Letters after distances represent depth of trawls;
s=shallow -6 m, m=medium-30 m, d=deep-110 m. Trawl data collected by National
Marine Fisheries Service.
51
•
Loligo 1993 .
I --
[ill Loligo 1994
depth (m)
I
600
2000
1800
~
"'....
l:::
1- 500
1600
"-..
I
1400
c. 1200
"' 1000
-.;
- 4ooE'
'
~
0
-5
c.
300 ~
E
.9
1- 200 0
I
::t
·:;
""'
800
:.s
.s
600
""
400
/
200
I
I
~
r-'
N
I
00
1-
lfl
IIIII
0
\0
0
N
~
0
N
.0
100
0
~
Davenport
Pescadero
Figure 9. Distribution and abundance of market squid, Loligo opalescens, in trawls along
Davenport and Pescadero transects. Bars represent abundance and lines represent bottom
depth (m). Numbers on x-axis are distance from shore (km) of the trawl station. Letters
after distances represent depth of trawls; s=shallow -6 m, m=medium-30 m, d=deep-110
m. Trawl data collected by National Marine Fisheries Service.
52
1!11 S. jordani 1993
I --
[J S. jordani 1994
600
600
- 500
I
"'
J:;
0
c.
400
""
- 4oos
~
-5
c.
I
-;;"'
·:;:"
-c 300
r- 300~
E
0
:a
100
0
g
r- 200 .D
I
.5 200
""
I
700
'"§: 500
...
depth (m)
I
/
_/
'"'
[]
/
j
E
Davenport
""E
r 100
I-
!Ill
E
0
E
Pescadero
Figure 10. Distribution and abundance of shortbelly rockfish, Sebastes jordani, in trawls
along Davenport and Pescadero transects. Bars represent abundance and lines represent
bottom depth {rn). Numbers on x-axis are distance from shore (krn) of the trawl station.
Letters after distances represent depth of trawls; s=shallow -6 m , m=medium-30 m,
d=deep-11 0 m. Trawl data collected by National Marine Fisheries Service.
53
100
1.0
80
~
Electivity
Index
-~ 80
0
.s
40
.5
~
20
.,"><
0
..s
0
-20
.::
; -40
ill"
tJ
-.5
r:
'""'
.s
-60
~-80
1.0
-100
~
"
"c.
::l
0
....,_
~
'
.~
:;
"
bll
c::
t.Ll
·a
.,"'....
0
-~
~
2:!
~
"
..0
~
::l
·c;
u
::l
"C
"
~
.,"
:.a"'
0
-~
:3
c.
8u
»
~
"
rn
~~
"
"'
·;;;,
"'
>-.
"0
-"'
"'
P'l
ci.
--;l
~
~
~
"
..0
"'
~
~
"
rn
::l
r:
bll
c::
tLl
""
v
"'
X
IS
M
Figure 11. The proportion of species in the chick diets compared with the proportion
in the trawls for Aiio Nuevo Island in 1993. Ignore negative signs on"% in Trawls"
y-axis. The line represents Ivlev's (1961) electivity index values. Engraulis-A,J =
Adults and Juveniles combined, L =Larval only.
54
100
~
i5"
.9
~
80
1.0
Electivity
Index
60
40
.5
20
0
-20
"' -40
~
~
·=
-60
~
-80
-1.0
-100
:.0"'
0"'
0
u
c..
"'
0
c:
:a
a
IZl
~
-,
.,:
'
-~
0
t:!
Oil
c:
IJJ
fl
·a -l' "' " " ..c:
"" "0"' ii:"'
"' 'b"o
"' 0 ..c:
""
"8
:.0
..c:
"' ._,
~
0
u
c.. 0
.$
't::
g "C
"'"' "' t1,
.0
..c:
c:
..c:
~
...:
"'
IJJ
IZl
ci.
"'
.~
~
~
;>,
OJ
~
6 "'
"'"'
CQ
.0
OJ
IZl
()
;>,
~
...,
-o
"'
·p
u
OJ
c:
2:::>
OJ
0::
"'
·;::;
"'
~
u
v
~
s""'
-.:
"'
OJ
""
Figure 12. The proportion of species in the chick diets compared with the proportion
in the trawls for Afio Nuevo Island in 1994. Ignore negative signs on"% in Trawls"
y-axis. The line represents Iv1ev's (1961) electivity index values. Engraulis-A,J =
Adults and Juveniles combined, L =Larval only.
55
200
180
180
160
~
~
"'0
u 160
140 "0"'c:
...
·;:;
~
c:
-
0
140
120
.s 120
..::!
;::!
0.
100
OJl
80
i£
c:
'6
0
0
60
.;
40
...
c:l
"'
~
20
0
.§
"0
~
100 "'::::
80
••••
..........-:~: ....... ~
60
;::!
c:l
.....0
...
a
z
0
40
20
?
g
..D
;::!
0
82 83 84 85 86 87 88 89 90 91 92 93 94 95
Figure 13. Rhinoceros Auklet population trends on Afio Nuevo Island. Closed circles (•)
represent the estimated number of breeding birds (50% occupancy rate applied to burrow
censuses before 1993). Open diamonds (0) represent the number of natural burrows open
during the breeding season (censuses conducted only once in 1986, 1987, and 1989).
Birds were seen flying with fish over the colony in 1982 but no burrow census was
conducted.
56
0.9
47
~
~
·a
0.8
:u0..
0.7
160
0..
0
SEFI
II
ANIBoxes
D
ANIBurrows
17
'0
(.)
01J
'0
c"'
<n
""'
.~
.<:
V>
-..]
'!"''l:.
..:::\
c.-
"'
n:
::;
'0
p
1993
1994
1995
1986-95
Figure 14. Rhinoceros Auklet productivity (number of chicks fledged per breeding pair) on At'io Nuevo
Island (ANI) and Southeast Farallon Island (SEFI) for 1993-1995. In 1995, productivity was determined
for pairs breeding in natural buiTows on ANI. Mean productivity on SEFI for 1986-1995 is presented.
Error bar is± one standard error. Numbers above bars are sample sizes. SEFI data collected by Point
Reyes Bird Observatory