744
Short
Communications
andCommentaries
ervation of avian blood and tissue samplesfor
DNA analysis.Can. J. Zool. 69:82-90.
SHERMAN,
P. W. ANDM. L. MORTON.1988. Extra-pair
fertilizationsin mountain White-crownedSparrows. Behav. Ecol. Sociobiol.
22:413-420
[Auk,Vol. 111
predominantlymonogamousbird: Genetic evidence. Anim.
Behav. 35:877-886.
WESTNEAT,
D.F. 1990. Geneticparentagein the Indigo Bunting:A study using DNA fingerprinting. Behav. Ecol. Sociobiol. 27:67-76.
SH•N,H.-S., T. A. BARGIELLO,
B. T. C•a•ac, F. R. JACKSON, WESTNEAT,D. F., P. W. SH•RMAN, AND M. L. MORTON.
AND M. W. YOUNG. 1985. An unusual coding
sequencefrom a Drosophilaclock gene is conserved in vertebrates.
Nature
317:445-448.
SMITH, H. G., R. MONTGOMERIE,T. POLDMAA, B. N.
WHITE,ANDP. T. BOAG.1991. DNA fingerprinting reveals relation between tail ornaments and
cuckoldryin Barn Swallows,Hirundorustica.Be-
1990. The ecology and evolution of extra-pair
copulations in birds. Curt. Ornithol. 7:331-369.
WETTON, J. H., R. E. CARTER,D. T. PARICIN,AND D.
WALTERS.
1987. Demographicstudyof a wild
HouseSparrowpopulationby DNA fingerprinting. Nature 327:147-149.
Received
30 June1993,accepted
17 November
1993.
hav. Ecol. 2:90-98.
WESTNEAT,
D. F. 1987. Extra-pair fertilizations in a
The Auk 111(3):744-748, 1994
Impact of InterspecificAggressionand Herbivory by Mute Swanson
Native Waterfowl and Aquatic Vegetation in New England
MICHAEL R. CONOVER • AND GARY S. KANIA 2
•Berryman
InstituteandDepartment
of Fisheries
andWildlife,UtahStateUniversity,
Logan,Utah 84322,USA;and
2Depart:aent
of WildlifeManagement,
NationalRifleAssociation,
11250WaplesMills Rd.,
Fairfax,Virginia22030,USA
One of the greatestthreats to American avifauna is
the establishmentof free-rangingexoticavian populations(Temple 1992).Despite evidenceof substantial harm causedby exoticbirds, most exotic species
are so poorly studiedthat many of their alleged environmental impactsremain largely undocumented
(Temple 1992). One such exotic speciesis the Mute
Swan (Cygnusolor),a Eurasianspeciesthat hasbeen
introducedseveraltimes into North America, beginning in the late 1800s(Allin et al. 1987).By the 1970s,
free-ranging populationsexistedin Michigan, Minnesota,Wisconsin,Wyoming, British Columbia, On-
whether theseconcernsarereal or whether swanpopulationsshouldbe controlled.At present,Mute Swans
are protectedin somestatesand unprotectedin others;in still others,governmentemployeesattemptto
control swan populationsby shaking eggs and removing adults (Allin et al. 1987). In this study, we
examined interspecificaggressionby free-ranging
adult Mute Swanswith breeding territoriesin freshwater ponds and the impact of their herbivory on
aquatic vegetation.
Methods.--Territorial pairs of free-ranging Mute
Swanswere observedat 15 freshwaterponds(2 to 30
tario, and in Atlantic coastalstatesfrom Maryland to
Massachusetts(Allin 1981). In the Atlantic Flyway,
free-rangingpopulationsincreasedfrom 200 birds in
1955 to 5,300 in 1987 (Allin et al. 1987).
Somebiologistsare concernedthat the increasing
ha) in New HavenCounty,Connecticutfrom 1982to
1987.Thesepondsrepresentedall freshwatersitesin
the studyarea known to have nestingpairs of freeited to freshwater
sites because these were the main
populationof free-rangingMute Swansmayhavean
areas where
Swans came into contact with
adverse impact on native waterfowl, owing to the
swan'saggressivenature (Reese1975,Williams 1989).
Swanssometimesattackother waterfowl, causinginjury or death (Stoneand Marsters1970, Willey and
Halla 1972,Allin et al. 1987).Furthermore,aggressive
swansmaydisplaceotherwaterfowl (Willey and Halla 1972). An additional concernis that the foraging
behavior of swansmay adverselyaffectaquaticplant
biomass,reducing the food available for other waterfowl. Currently, data are insufficient to judge
tive waterfowl.
ranging Mute Swansin 1982. Observationswere limMute
na-
Data on the impactof interspecificaggressionwere
collectedon both membersof a swan pair simultaneously;data for malesand femaleswere analyzed
separately.Sexof pair memberswas determinedby
the larger body size and larger fleshyknob on the
forehead of males (Bellrose 1980). Pairswere observed
year-round for 30-rain periods, randomly selected
amongdaylighthours.Observations
were madefrom
shore,usuallyfrom a carto minimizedisturbance
of
July 1994]
ShortCommunications
andCommentaries
the birds.During an observationperiod,we notedall
aggressivebehaviorsby swans,the other speciesin-
volved,and the outcomes.Agonisticbehaviorsby
swansincluded threats (raising wings above back),
chases,and physical attacks.All sites were searched
weekly from a boat or shorefor nestingwaterfowl.
Samplesizesfor all statisticaltestswere the number
of subjectsunder observation.Paired t-testswere used
to ascertainif there were sexualdifferencesin aggressivebehavior. To test whether seasonalvariation
745
T^BLE1. Number of aggressive
interactionsper hour
(ϖ St) initiatedby free-rangingmale(n = 15)and
female (n = 15) Mute Swans against swans and
other waterfowl
at 15 freshwater
sites in Connect-
icut.
Speciesbeing
attacked
Mute Swan
Mallard
American
Black
Duck
Canada Goose
Human
Male
0.62 ñ 0.21
0.10 ñ 0.03
Female
0.31 ñ 0.11
0.07 ñ 0.01
occurredin swanaggressiveness,
dataon eachswan's
0.03 ñ 0.01
0.01 ñ 0.001
behaviorduring March to May, Juneto August,and
0.38 ñ 0.29
0.01 ñ 0.01
Septemberto Februarywere analyzedusinga two0.25 ñ 0.07
0.16 ñ 0.06
wayANOVA (seasons
vs.subjects).
Linearregressions Total interspecific
were usedto compareinterspecificaggression
rates
aggression
0.80 ñ 0.30 0.31 ñ 0.09
of individualswansto their intraspecificaggression
rates.
pa
0.03
0.07
0.10
0.11
0.08
0.04
Paired one-tailedt-test comparingmalesand female swans.
To determinewhether waterfowlwere avoiding
areas near swans, we prepared a map of each site
noting water depths.At the beginningof eachobservation period, we noted a swan's location and ran-
site was consideredas being composedof five sections. Eachtime a site was sampled,all of the abovegroundvegetationwasharvestedwithin a 0.33 x 0.33
domlyselectedfromthe mapanotherpoint of similar m sampling frame placed within each section. Subdepth.At 5-rainintervals,we thenvisuallyestimated sequent sampling was taken elsewhere in each secthe distancefromthe nearestindividualof everywa- tion so that no area was harvested more than once.
terfowl speciespresent to both the swan and the ranSamplingwas conductedin late May or early June
domlyselectedpoint. For eachindividual swan,these when the exclosures
were established(year1,spring),
datafrom all observationperiodswere then combined three months later (year 1, fall), and 15 months later
to yield a single mean distancevalue between it and (year 2, fall). Vegetationwas segregated
by species,
each waterfowl species.A similar mean value was driedfor morethan24 h at 110øC,
andweighed.Paired
obtainedfor thepairedrandompoints.Hence,sample t-testswere usedto comparethe biomassof grazed
sizes consisted of the number of individual swans
and ungrazedsites.One grazedand one ungrazed
under study. A paired t-test was then used to test samplewere obtainedfrom eachpondwith eachsamwhethermeandistances
betweena waterfowlspecies ple consistingof 10 sampleframescollectedfrom the
and a swan differed from the distance between that
samepond (five sampleframesper exclosurex two
speciesand a random point.
exclosuresper pond). Hence, for thesestatisticaltests,
The experimenton the impactof swanherbivory the samplesize equalledthe number of pondsunder
wasconductedat 12 of the freshwaterpondsusedin
the other part of this study.Exclosures
were erected
in late May or early June at two ponds in 1983,six
study.
Results.--During405.0h of observationon 15 males
ponds in 1984, two in 1985, one in 1986, and one in
interactionsby free-ranging Mute Swans;410 were
1987.At each pond, we locatedtwo sites(3 x 3 m
each)between20 and 50 m apartthat were similarin
species.Swans were aggressivetowards humans 174
and398.5h on 15females,we observed870aggressive
directedagainstother swansand 460 againstother
water depth (between0.5 and 1.0 m) and appeared times (usuallyinvolving a threat displaydirectedat
to have similar vegetation.One of these siteswas
randomly selectedto serve as an open site where
peopletrying to feed them). They were alsoaggressivetowardsGreat Blue Herons (Ardeaherodias)
three
swanscould graze(hereafterreferredto as grazed times, gulls (Larusspp.) three times,scaups(Aythya
site)and markedwith a woodenpost.The othersite spp.) twice, Gadwalls (Anasstrepera)once, domestic
served as the ungrazed site and a 3 x 3 m swan ducksseventimes,and domesticgeese21 times.On
exclosure was centered on it. The exclosure was built
all otheroccasions,
interspecific
aggressive
behavior
by attachinga 0.3 to 0.5 m wide chicken-wire fence was directedat Mallards(Anasplatyrhynchos),
Amerto eight posts.The fence was built so that its bottom icanBlackDucks(A. rubripes),
andCanadaGeese(Branta
edgewas level with the water surfaceso that it would
excludeswansbut not other aquaticherbivoressuch
as fish, turtles,diving ducks,and muskrats.Another
pair of grazedand exclosuresiteswas setup on the
oppositesideof the pond.Datafrom thesetwo pairs
werealwayscombinedfor analysisto providea single
grazedand ungrazedvalue for eachpond.
Forvegetationsampling,eachgrazedandungrazed
canadensis;
Table 1).
There were no significantdifferencesamongyears
in the aggressivebehavior of swans.Hence, data were
summarizedfor eachindividual subjectacrossyears
and statistical tests conducted on the combined data.
Males were more aggressivethan females towards
both conspecifics
and other species(Table 1). Rates
of interspecificand intraspecificaggressionwere not
746
ShortCommunications
andCommentaries
chasingthem (Table 3). Swans got closeenough on
50 occasions
to bite individuals of other species.Mal-
TABLE2. Mean number of aggressiveencountersper
hour initiatedby free-rangingmaleand femaleMute
Swans during different seasonsat five freshwater
lards, American Black Ducks, Canada Geese, and do-
sites in Connecticut.
mesticgeesewere bitten, but injuries did not appear
serious.Once a swan used its wing to strike a goose.
CanadaGeesewere observednestingor with young
at 7 of the 15 study sites, Mallards at 12 sites, and
Season
Sep-
Speciesbeing
attacked
Male
Canada
Human
American
March- June- temberMay August February
0.72
0.24
0.05
0.25
0.24
Goose
0.42
0.04
0.19
0.06
0.51
Black Ducks
at 1 site. We did not observe
any instanceswhere swansfoiled a nesting attempt
by anotherwaterfowl species.Despitethe swans'aggressivenature, there was no evidence that any waterfowl speciesavoided areasnear swans.Distances
between the closestwaterfowl to a random point and
to a swan did not vary for most waterfowl species
swans
Mute Swan
Mallard
American
Black Duck
[Auk, Vol. 111
1.70 •
0.11
0.03
0.14
0.05
(Table 4).
Total interspecific
When the exclosureswere erectedin May to assess
the impact of swan herbivory, above-groundplant
Female swans
biomassfor all plant speciesand for the three most
0.47
0.01
0.69
Mute Swan
ubiquitousspeciesdid not differ betweengrazedand
0.07
0.05
0.09
Mallard
ungrazedsites(Table5). In the fall, there wasslightly
0.05
0.00
0.14
American
Black Duck
moreplant biomassin the exclosuresthan in the grazed
0.23
0.09
0.14
Human
sites,but the differenceswere not statisticallysignifTotal interspecific
icant.During the winter, exclosures
at five pondswere
0.35
0.21
0.26
interaction
destroyedby shifting ice or vandalism.At the re.' None of differencesamongseasons
were significant(P < 0.05)based
maining sevenponds,plant biomassin the fall of the
on two-wayANOVA (2,8 df).
secondyear wasslightly higher in the exclosuresthan
at the grazed sites,but the differenceswere not statistically significant.
correlated in either males (r 2 = 0.01, P = 0.80) or
Discussion.--In
southernEngland,someMute Swans
females (rz = 0.01, P = 0.76).
occupyand defend their territories year-round, deSwansoccupiedtheir territoriesthroughout the year pending uponwinter weatherand availability of food
exceptwhen pondsfroze.Five malesand five females resourceson the territory (Scott 1984). Free-ranging
were observedoften enough in each season(more Mute Swans in Connecticut also occupied their terthan 10 observationperiods/season)to be included ritories almostyear-round (exceptduring midwinter
in the seasonalanalysis.Aggressionrates for both when freshwater sites became frozen). Whenever
male and female swansdid not vary by season(Table present on their territories, Mute Swans engaged in
both interspecificand intraspecificaggression.Con2). Males varied in their aggressiveness
toward American BlackDucks,CanadaGeese,and all interspecific sequently,they threatenedor attackedboth breeding
interactions combined; females did not (Table 2).
waterfowl and thosemigrating through or wintering
0.79
interaction
0.91
•0.40
Aggressiveinteractionsusuallyendedwhen ducks
moved a few metersaway from a threatening swan.
However, Canada Geese often had to swim more than
50 m, had to fly away, or were chasedinto water less
than 5 cmdeepor onto dry land beforea swanstopped
in Connecticut.
Mute Swans spent as much time in interspecific
aggressionas in intraspecificaggression.Intensive
interspecificaggressionis rare among mostbirds, but
is common in one group: steamer-ducks(Tachyeres
TABLE
3. Differentresponses
by waterfowlto aggressive
encounters
with free-rangingmaleor femaleMute
Swans at 15 freshwater
sites in Connecticut.
Percent of all waterfowl responses
Species
being
attacked
Moved
n
< 10 m
Ducks
Canada Goose
75
115
55
30
Ducks
Canada Goose
34
29
74
69
10-50 m
Male
24
0
Flew
ashore
far side
away
3
21
7
11
3
10
4
4
0
14
3
7
0
0
0
10
swans
29
24
Female
Swam Flew
to
>50 m
swans
July1994]
ShortCommunications
andCommentaries
TABLE
4. Distance(œ+ SE) betweena free-ranging
Mute Swan (or a random point) and closestwaterfowl to them at 15 freshwater sites in Connecticut.
Speciesbeing
attacked
Distance (m) to
n
(sites)
Swan
Male
Mallard a
13 b
Randompoint
7
5
4
4
49.3 + 9.1
60.0 + 7.5
34.7
46.2
57.8
82.8
57.0
58.2
53.5
81.2
Female
Mallard
14
TABLE5. Above-groundplant biomass(g/m 2) of all
plantsand for threeubiquitousspeciesat siteswhere
Mute Swanscouldgrazeand at ungrazedsitesthat
were surroundedby exclasureswhen first erected
(Year 1, Spring;n = 12 ponds),three monthslater
(Year 1, Fall; n = 12 ponds), and 15 months later
(Year 2, Fall; n = 7 ponds).
swans
American
Black Duck
Wood Duck
Other ducks
Canada Goose
+
+
+
+
9.4
15.6
13.9
27.4
+
+
+
+
19.5
18.3
22.4
34.1
swans
51.6 + 10.4
Sampling
period
Year I, Spring
Year 1, Fall
Year 2, Fall
61.9 + 10.8
American
Year 1, Spring
Black Duck
Wood Duck
Other ducks
Canada Goose
9
6
6
4
33.6
27.8
69.5
82.5
+
+
+
+
11.5
5.6
22.6
19.9
43.6
39.9
42.3
109.8
+
+
+
+
10.5
16.3
6.7
30.3
"The only significantdifference(P < 0.05) basedon paired t-tests
(two-tailed) was distance between American Black Ducks and male
swans.
at all sites.
spp.; Livezey and Humphrey 1985, Murray 1985,
Nuechterlein and Storer 1985). This raisesthe question of whether
steamer-ducks
Biomass(g/m 2)
Grazed
Ungrazed
All plant species
36.0
37.9
59.1
75.4
t'
0.80
40.2
60.9
1.38
0.62
Potomogeton spp.
1.8
7.3
1.00
Year 1, Fall
1.5
0.1
1.19
Year 2, Fall
0.1
0.0
1.00
Nymphaea odorata
Year 1, Spring
2.3
1.2
1.47
Year 1, Fall
Year 2, Fall
1.45
1.23
12.3
12.6
16.2
18.3
EIodea canadensis
• Samplesizesare lessthan 15 becausesomewaterfowl specieswere
not observed
747
Year I, Spring
1.0
1.0
0.00
Year I, Fall
Year 2, Fall
4.6
1.6
3.4
1.6
1.01
0.00
All comparisonsnonsignificant(P > 0.05).
and swans share char-
acteristicsthat can explain their intraspecificaggression. Steamer-ducks have massive heads, thick skin,
and a large size advantage over their interspecific
opponents;these traits reduce the risk of defeat or
injuryfrom interspecific
aggression
(Nuechterleinand
Starer 1985). Likewise, swans have a large size advantage over their interspecificavian opponentsand
facelittle risk of injury during their attackson other
birds. None of the birds we observed being threatened or attackedby swansfought back.
Advantages
steamer-ducks
or Mute Swansgainfrom
interspecificaggressionare unclear.Nuechterleinand
Starer (1985) proposedthat male steamer-ducksexhibit interspecificaggressionto displaytheir fighting
ability to females and to reduce food competition.
Livezey and Humphrey (1985) argued that the advantageswhich steamer-ducks
gainfrom interspecific
that shared a pond with a Mute Swan was the energetic costof evasion.There was, however, considerable variation among individual swansin the tolerance of other waterfowl. The more aggressiveones
chasedother waterfowl, especiallygeese,that were
in their vicinity. The more tolerantswansdid not.
The swans'interspecificaggressionalsocould ad-
verselyimpactother waterfowlby interferingwith
breedingattempts,but suchwasnot observed.Unlike
Kania and Smith (1986), we were unable to document
any incidentsof a swancausinga nestingattemptto
fail.
Nuechterleinand Starer(1985)reportedthat steamer-duckswere able to excludemolting Red Shovelers
(Anas platalea)from their territories. We found no
indication
that the distances between
a swan differed
waterfowl
from the distance between
and
waterfowl
aggression
mayincludeprotectionof young,defense and a randompoint. If waterfowl were avoiding areas
of food from marginalcompetitors,sexualadvertise- near Mute Swans, a nonrandom distribution would
have been expected,but such was not found. One
Biologistsare concernedthat the expandingpop- difference between our observations and those of
ulationsof Mute Swansin the United Statesmay have Nuechterlein and Starer (1985) is that we did not limit
an adverseimpacton native waterfowl either because our observationsto molting waterfowl. The birds in
of theiraggressive
natureor theirherbivoryonaquat- our studycould easilyescapefrom a swan if threatic plants (Reese 1975, Allin et al. 1987, Kania and ened. However this would not be true when geese
Smith 1986,Williams 1989).We found that Mute Swans were molting and unable to fly. Hence, aggressive
engagedin high rates of aggressionagainstnative Mute Swansmay excludemolting geesefrom their
ment, and practice for intrageneric combat.
ducks and geese, but that these episodesconsisted
territories.
mainly of threatsand chases,and rarely resultedin
physicalcontact.Hence,onecostfor nativewaterfowl
tive waterfowl if their herbivory impactedaquatic
Swansalso could have a deleterious impact on na-
748
Short
Communications
andCommentaries
plant biomass,thereby reducing either the vegetative
or macroinvertebrate (Krul11970) food resourcesused
by native waterfowl. In this study, we were unable
to documentany effectof swan herbivory on aquatic
vegetation. However, swan populations in the area
are increasing(Allin et al. 1987)and breeding territories are becoming smaller. For instance, in 1983
there were two breedingpairson Lake Whitney, but
by 1990 there were six. Hence, while current swan
densitiesare not having an impact on aquaticplant
biomassin New Englandponds,this may changeif
swan densitiesincreasesubstantially.
Acknowledgments.--We
thank P.M. Piconeand B.
Young for their help in collecting these data.
LITERATURE CITED
ALLIN,C.C. 1981. Mute Swansin the Atlantic Flyway. Proc.Int. Waterfowl Sympos.4:149-152.
ALLIN, C. C., G. G. CHASKO,AND T. P. HUSBAND. 1987.
[Auk,Vol.111
KRULL,J. N. 1970. Aquatic plant-macroinvertebrate
associations
and waterfowl.J.Wildl. Manage.34:
707-718.
LIVEZEY, B.C., AND P.S. HUMPHREY. 1985. Territo-
riality and interspecificaggressionin steamerducks. Condor
87:154-157.
MURRAY,B. G., JR. 1985. Interspecificaggressionin
steamer-ducks.
Condor
87:567.
NUECHTERLEIN,
G. L., AND R. W. STORER.1985. Aggressivebehaviorandinterspecifickilling by Flying Steamer-Ducksin Argentina. Condor 87:8791.
REESE,
J.G. 1975. Productivity and managementof
feral Mute Swansin ChesapeakeBay.J. Wildl.
Manage. 39:280-286.
Scour,D.K. 1984. Winter territorialityof Mute Swans
Cygnusolor.Ibis 126:168-176.
STONE,
W. B., ANDA.D. MARSTEPS.
1970. Aggression
among captive Mute Swans. N.Y. Fish Game J.
17:50-52.
Mute Swansin the Atlantic Flyway: A review of TEMPLE,
S.A. 1992. Exoticbirds:A growingproblem
the history,populationgrowth and management
with no easysolution.Auk 109:395-397.
needs. Trans.
Northeast
Sect. Wildl.
Soc. 44:32-
47.
WILLEY, C. H., AND B. F. HALLA. 1972. Mute Swans
of RhodeIsland.RhodeIslandDep. Nat. Resour.,
BELLROSE,
F. C. 1980. Ducks, geese, and swans of
North America, 3rd ed. StackpoleBooks,Harrisburg, Pennsylvania.
Div. Fish Wildl.
Pam. 8.
WILLIAMS,
W. 1989. Dark side of a classicbeauty.
Natl. Wildl. 27(2):42-49.
KANIA, G. S., AND H. R. SMITH. 1986. Observations
of agonistic interactions between a pair of feral
Mute Swansand nesting waterfowl. Conn. War-
Received15 July 1993,accepted
13 December
1993.
bler 6:35-37.
The Auk 111(3):748-750, 1994
Siblicide
and Cannibalism
at Northern
Goshawk
Nests
CLINT W. BOALAND JOHNE. BACORN
School
of Renewable
NaturalResources,
University
of Arizona,Tucson,
Arizona85721,USA
In many asynchronouslyhatchingbirds,broodsize
decreasesduring the nestling period in a characteristicpattern, starting with the last-hatchedchick. This
has been widely interpreted as a systemby which
family size is adjustedto match available levels of
essentialparentally provided resources(Lack 1954).
One causeof this mortality in someraptorsand other
predatorybirdsis fatal sibling aggression(e.g.O'Connor 1978, Stinson 1979, Mock et al. 1990), after which
the victim'stissuesare sometimesingestedby family
members(Ingram 1959, Mock 1984, Bortolotti et al.
1991).There is somecontroversyover how often consumptionof the victim occursand, asa separateissue,
how importantan evolutionarycomponentthe cannibalism per se may be (reviews in Elgar and Crespi
1992,Stanbackand Koenig 1992).
Siblicideandcannibalismisuncommonamongavian species(Mock 1984,Mock et al. 1990,Stanbackand
Koenig 1992). Most reports are based on indirect ev-
idence,such as remainsof nestlingsfound in nests
(Heintzelman 1966, Pilz 1976, Moss 1979, Bechard
1983),andmayfail to unequivocally
identifythe cause
of death. Observational
accounts of siblicidal
and can-
nibalistic events are few (Newton 1978, Pilz and Seibert 1978, Jonesand Manez 1990, Bortolotti et al. 1991,
Negro et al. 1992).The key events tend to be brief
(Mock 1984) and may go unwitnessedunlessa nest
is under constant observation.
We report the observationof a nestling Northern
Goshawk(Accipiter
gentilis)killing and cannibalizing
a siblingafterthe adult femaledisappeared
from the
nestarea.In addition, we describea separateincident