The Condor 111(3):462–469
¡The Cooper Ornithological Society 2009
MICROCLIMATE OF NATURAL CAVITY NESTS AND ITS IMPLICATIONS
FOR A THREATENED SECONDARY-CAVITY-NESTING PASSERINE
OF NEW ZEALAND, THE SOUTH ISLAND SADDLEBACK
BRYAN R HODES1,3, COLIN O’D ONNELL2 ,
2
AND I AN JAMIESON1
1
Department of Zoology, University of Otago, P. O. Box 56, Dunedin 9001, New Zealand
Research, Development, & Improvement, Department of Conservation, Christchurch, New Zealand
Abstract. Secondary-cavity-nesting birds occur widely throughout the world, but little information is
available on the benefits of the nest’s microclimate for such species, particularly for those using natural cavities.
We investigated the influences of microclimate on a threatened secondary-cavity-nesting passerine, the South
Island Saddleback (Philesturnus carunculatus carunculatus). Our aims were to determine whether (1) saddlebacks select tree cavities with microclimates less variable than those of other tree cavities in their surrounding
territory, (2) whether structural aspects of tree cavities translate into certain microclimate characteristics, and
(3) if less frequently used sites not in tree cavities (e.g., cavities in banks or in vegetation) have thermal properties similar to those of tree-cavity nests. We found that the saddleback’s tree-cavity nests were more stable in
temperature, more insulated against cold, and did not change temperature as rapidly as the ambient air or unused tree cavities. Regression analysis showed that of structural characteristics of tree cavities examined, only
one, entrance width, was significantly associated with an aspect of microclimate (minimum temperature). Additionally, we found that regardless of cavity type the thermal properties of saddleback nest cavities were similar. These results indicate that saddlebacks likely select nest cavities with less variable thermal properties that
are potentially beneficial, and future studies experimentally manipulating the variability of microclimate may
be fruitful in determining the effect of microclimate on reproductive success. Nevertheless, this study is one
of the first to demonstrate microclimate as a factor determining selection of natural nest cavities over available
unused cavities.
Key words: facultative cavity nesting, hole nests, nest temperature, non-tree cavity nests, Philesturnus
carunculatus, South Island Saddleback.
Microclima de Nidos en Cavidades Naturales e Implicancias para Philesturnus carunculatus
carunculatus, una Especie no Excavadora de Cavidades y Amenazada de Nueva Zelandia
Resumen. Las especies de aves no excavadoras que nidifican en cavidades se distribuyen ampliamente en
el mundo, pero se sabe muy poco sobre los beneficios del microclima del nido para estas especies, particularmente para aquellos que utilizan cavidades naturales. Investigamos las influencias del microclima sobre Philesturnus carunculatus carunculatus, una especie paseriforme no excavadora que nidifica en cavidades. Nuestros
objetivos fueron determinar si (1) los individuos de P. c. carunculatus seleccionaron las cavidades en árboles
con menor variación microclimática con relación a las demás cavidades existentes en el territorio circundante,
(2) los aspectos estructurales de las cavidades se tradujeron en determinadas características microclimáticas y
(3) los sitios utilizados con menor frecuencia que no se encuentran en cavidades en árboles (e.g., cavidades en
barrancos o en la vegetación) tienen propiedades térmicas similares a las de los nidos en cavidades en árboles.
Encontramos que los nidos de P. c. carunculatus en cavidades tuvieron temperaturas más estables, tuvieron un
mayor aislamiento contra el frío y no variaron de temperatura tan drásticamente como la temperatura ambiente
o de las cavidades no usadas. Los análisis de regresión mostraron que de las características estructurales examinadas, sólo la entrada de las cavidades en árboles estuvo significativamente asociada con un aspecto del microclima (temperatura mínima). Además, encontramos que independientemente del tipo de cavidad, los nidos
de P. c. carunculatus tuvieron propiedades térmicas similares. Nuestros resultados sugieren que esta especie
selecciona cavidades con propiedades térmicas menos variables, que son potencialmente beneficiosas. Estudios
futuros que manipulen experimentalmente la variabilidad del microclima podrían ser útiles para determinar
los efectos del microclima sobre el éxito reproductivo. Sin embargo, este estudio es uno de los primeros en demostrar
que el microclima es un factor importante en la selección de cavidades naturales con relación a las cavidades
disponibles no utilizadas.
Manuscript received 12 September 2008; accepted 10 June 2009.
3
E-mail: [email protected]
The Condor, Vol. 111, Number 3, pages 462–469. ISSN 0010-5422, electronic ISSN 1938-5422. ‘2009 by The Cooper Ornithological Society. All rights reserved. Please direct
all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/
reprintInfo.asp. DOI: 10.1525/cond.2009.080030
462
EFFECT OF NEST MICROCLIMATE ON THE SADDLEBACK
INTRODUCTION
For successful reproduction, avian nests have three microclimatic requirements: the appropriate temperature, humidity,
and composition of respiratory gas. Temperature, however,
is regarded as the component most crucial for successful reproduction in birds (Walsberg 1980, Wachob 1996, Kern and
Cowie 2000, Ar and Sidis 2002, Lill and Fell 2007). Given
that temperature influences reproductive success, birds are expected to take steps to select sites that provide a favorable microclimate (van Riper et al. 1993, Gloutney and Robert 1997,
Burton 2006, Tieleman et al. 2008). Numerous aspects of nest
sites have been shown to contribute to a favorable microclimate, including levels of solar radiation, thickness of tree
walls, density of surrounding vegetation, orientation, cavity
size, bark type, and exposure to rain (Wachob 1996, Hooge et
al. 1999, Reid et al. 2000, Rauter et al. 2002, Radford and Du
Plessis 2003, Ardia et al. 2006).
Cavities have long been known to provide energetic benefits to birds (Kendeigh 1961). Birds save substantial energy
by roosting in cavities, especially during inclement weather
(Caccamise and Weathers 1977, Du Plessis et al. 1994, Du
Plessis and Williams 1994, Cooper 1999). The advantages of
roosting within a cavity are likely to translate to breeding in a
cavity as well, as reproduction is one of the periods most energetically costly for birds (Gustafsson et al. 1994). Energy
balance is critical during reproduction, as birds must incubate
eggs to a temperature sufficient for embryonic development
and minimize thermoregulatory costs to nestlings (Webb
1987, Wachob 1996, Reid 2000, Dawson et al. 2005).
Studies have shown that cavity-nesting birds benefit reproductively from a favorable microclimate, but these studies
are largely restricted to nest boxes (Blem and Blem 1994, Wachob 1996, Stamp et al. 2002), which may confound results,
as nest boxes do not replicate all of the characteristics of natural nests (McComb and Noble 1981, Møller 1989). Results
of previous studies indicate that microclimate can influence
the reproductive success of species nesting in natural cavities,
but these studies are largely limited to primary-cavity-nesting
birds (Hooge et al. 1999, Wiebe 2001). Because they excavate
their own nests, primary-cavity-nesting birds likely are less
constrained during cavity selection than secondary-cavitynesting birds, so those studies of microclimate selection are
likely less informative (Wiebe 2001). Studies of microclimate
in natural cavity nests of secondary-cavity-nesting birds are
lacking (Albano 1992, Wachob 1996). Natural cavities seem to
vary in quality, but whether secondary-cavity-nesting birds can
and do select natural nest cavities for microclimatic benefits remains uncertain (Kesler and Haig 2005, Paclík and Weidinger
2007). We address this issue by asking: Do microclimates of
natural nest cavities selected by the secondary-cavity-nesting
South Island Saddleback (Philesturnus carunculatus carunculatus) differ from unused cavities within their territories?
Do particular characteristics of nest cavities translate into
463
specific microclimate characteristics? And do cavity nests
not in trees provide thermal properties comparable to those of
tree-cavity nests?
METHODS
We studied a color-banded population of South Island Saddlebacks on Ulva Island, New Zealand (259 ha, 46n 56` S, 168n
08` E, highest point 74 m above sea level). To facilitate the
eradication of Norway Rats (Rattus norvegicus), a grid system
had previously been cut through the island’s understory vegetation, with tracks spaced at 100-m intervals and bait stations
placed along each track at 100-m intervals. We used these
tracks and bait stations to divide the island into 100-m 2 sections to search for breeding pairs of saddlebacks in the forest.
In the central part of the island the forest is mature temperate
podocarp–hardwood with the main canopy trees consisting
of totara (Podocarpus cunninghamii), miro (Prumnopitys ferruginea), rimu (Dacrydium cupressinum), and southern rata
(Metrosideros umbellata). This central forest is surrounded
mainly by coastal scrub dominated by leatherwood (Olearia
colensoi) and inaka (Dracophyllum longifolium).
The South Island Saddleback is a threatened poor-flying
passerine (family Callaeidae) endemic to New Zealand. Saddlebacks are facultative cavity nesters and nest in a variety
of substrates, including tree cavities (and nest boxes), ground
cavities, the crowns of tree ferns (Dicksonia squarrosa), leaf
litter in banks overlooking the ocean, clumps of vegetation,
or, occasionally, in open-cup nests in tree forks, but typically
saddlebacks use tree cavities (Hooson and Jamieson 2003a).
Like all passerines breeding in New Zealand, saddlebacks are
nonmigratory and remain in pair bonds year round. Saddlebacks have a limited ability to coexist with introduced terrestrial mammalian predators and have completely disappeared
from New Zealand’s main islands and many near-shore islands (Hooson and Jamieson 2003b, Wilson 2004) but were
reintroduced to Ulva Island in 2000 after rats had been eradicated (Lovegrove 1996, Hooson and Jamieson 2004).
For this study from September to February (austral summer) in 2006–2007 (hereafter abridged 2007) and again in
2007–2008 (hereafter abridged 2008) we located saddleback
nests by following pairs in territories established in previous
breeding seasons as well as those located in new territories.
We usually found nests as the female entered to incubate or as
both sexes returned to feed nestlings. We then marked the nests
with flagging tape within several meters and checked them
periodically (longest interval was 7 days) to establish approximate dates for banding nestlings. After the young had fledged,
we recorded numerous measurements for each tree-cavity
nest and also for the closest most suitable unused tree cavity.
We located the unused cavity by searching from the ground
for the nearest cavity with a nonvertical entrance 6 cm,
large enough to hold nest and bird, and dry inside. We classified saddleback nests as either bank cavities (holes in the
464
BRYAN RHODES
ET AL .
vegetation and ground of coastal bluffs), debris cavities (holes
in dead vegetation above the ground), or tree cavities, but we
measured only the more frequently used tree cavities. Treecavity measurements included vertical height and vertical
depth (both measured from the bottom of the entrance hole),
horizontal depth (measured from the bottom of entrance hole
to the back of the cavity), horizontal width (distance between
walls at the bottom of the entrance hole), the tree’s diameters
at cavity height (DCH) and at breast height (DBH), and height
of the entrance hole above the forest floor. If the cavity had
more than one entrance, measurements were taken at one of
the holes selected at random. We identified the tree’s species
and whether it was dead or alive.
We recorded microclimate data with Hobo ProSeries data
loggers (Onset Computer Corp., Pocasset, MA), which recorded
ambient temperature and the temperature inside each tree cavity. We recorded temperatures after the completion of first
clutches during both breeding seasons (8 January–13 February
2007 and 7–24 February 2008). To record ambient temperatures
we placed data loggers inside simple Stevenson’s screens (device to shield weather instruments from rain and direct sun) and
placed them equidistant between tree-cavity nests and unused
tree cavities at approximate mean nest height (~2 m). To record
cavity temperatures we first removed the nests so that temperatures would be equivalent to those birds would experience when
selecting nest sites. We then positioned data loggers in both nest
cavities and unused cavities at the level of the nest cup (or the
inferred level of the nest cup in unused cavities). Temperatures
of each type (nest, unused cavity, ambient) were simultaneously
recorded every 5 min over the same 5-day period. Because we
had only 10 data loggers, we placed them at nest sites as the first
nesting attempts were completed and then transferred them to
other available sites until data were recorded for all first-clutch
cavity nests. Additionally, during the 2008 breeding season we
recorded a sample of temperatures from cavity nest sites not in
trees (n 8; see classification above).
STATISTICAL ANALYSIS
We used a mixed (between and within subjects) ANOVA to
analyze temperature differences among tree-cavity nests,
unused tree cavities, and ambient air (Tabachnick and Fidell
2007). For each site, we averaged the temperature for each
hour (hr 1 00:00–00:55, etc.) and used these hourly measures to generate single descriptive temperature variables,
mean (Tmean: mean of all hourly averages), maximum (Tmax:
maximum of hourly average values), minimum (Tmin: minimum of hourly average values), range (Trange: equals Tmax –
Tmin) and rate (Trate: absolute value of hr2 – hr1, etc.) for each
24-hr day in the 5-day period. We analyzed each of the descriptive temperature variables in parallel mixed betweenand within-subjects ANOVA with each day (n 5) serving as
a repeated measure (within-subject effect) in the model. We
included site as a categorical independent between-subjects
TABLE 1. Comparison of means o SE for five temperature variables for nest cavities, unused cavities, and ambient air. Statistics and associated P values are generated
from mixed between- and within-subjects ANOVAs.
Variable and year
Mean
2007
Cavity a
Unused
Ambient
2008
Cavity
Unused
Ambient
Maximum
2007
Cavity b
Unused
Ambient
2008
Cavity a,b
Unused
Ambient
Minimum
2007
Cavity a,b
Unused
Ambient
2008
Cavity b
Unused
Ambient
Range
2007
Cavity a,b
Unused
Ambient
2008
Cavity a,b
Unused
Ambient
Rate
2007
Cavity a, b
Unused
Ambient
2008
Cavity a,b
Unused
Ambient
Mean
F2, 63
P
12.39 o 0.11
12.14 o 0.11
12.41 o 0.13
32.6
0.001
13.08 o 0.15
13.15 o 0.17
13.24 o 0.18
1.1
0.35
13.77 o 0.13
13.80 o 0.16
15.15 o 0.22
30.2
0.001
14.58 o 0.21
14.94 o 0.23
16.40 o 0.27
47.7
0.001
11.23 o 0.11
10.90 o 0.11
10.31 o 0.13
95.6
0.001
11.59 o 0.13
11.45 o 0.14
10.64 o 0.17
28.5
0.001
1.54 o 0.10
1.90 o 0.14
4.84 o 0.20
144.8
0.001
1.37 o 0.11
1.59 o 0.11
2.83 o 0.17
66.1
0.001
0.20 o 0.01
0.22 o 0.01
0.45 o 0.02
276.1
0.001
0.22 o 0.01
0.26 o 0.02
0.49 o 0.02
164.2
0.001
a
Difference between nest cavities and unused cavities
significant.
b
Difference between nest cavities and ambient air
significant.
variable. We analyzed years separately and included contrast
statements to compare differences among tree cavity nests,
unused tree cavities, and ambient temperature for each of the
five descriptive temperature variables (Table 1).
EFFECT OF NEST MICROCLIMATE ON THE SADDLEBACK
To analyze if characteristics of tree-cavity nests influenced the five descriptive temperature variables, we used a
stepwise multiple regression. However, we excluded some
nest-tree characters (see below) that were significantly correlated, and we also combined nests from 2007 and 2008 for
regression analysis, as none of these measurements in 2007
differed from those in 2008 (t –0.482–1.051, P 0.05). For
each nest we averaged the descriptive temperature variables
over the five 24-hr periods to generate a single value to serve
as the dependent variable in our analysis. We analyzed each
of these descriptive temperature variables singly in our stepwise multiple regression analysis against three characteristics (entrance hole’s width, cavity’s horizontal width, tree’s
DCH), which were entered into the model simultaneously. We
included variables in the model if P 0.05 and excluded them
if P 0.05.
We examined differences in microclimate among cavity
types by using repeated-measures linear mixed models (West
et al. 2007). To increase the power of this longitudinal analysis we combined the various types of cavity nests that were not
in trees into one category, as preliminary analysis revealed no
significant difference in the five temperature variables among
these nest types (P 0.10). A cavity nest not in a tree was then
selected and paired with the nearest tree-cavity nest to control for potential differences in habitat structure and exposure
across the island (Michel 2006). We included terms for cavity
type (1 tree cavity, 2 not tree cavity), day (1, 2, … 5) and
their interaction (cavity r day) as fixed factors in our model
and used a diagonal covariance structure for the repeated effects in the model. Day served as the repeated measure in our
model, and each of the five descriptive temperature variables
were analyzed separately. We used pairwise comparisons to
evaluate estimated marginal means for significant differences
between cavity types after performing Bonferroni adjustments for multiple comparisons, and the nominal significance
level before Bonferroni adjustment was P 0.05. We analyzed
all data with SPSS 16.0 (SPSS Institute, Inc., 2007).
RESULTS
Saddleback nests (n 84) were in a variety of substrates including banks (15.1%), debris piles (8.6%); one was an opencup nest (1.0%). Over the 2 years of the study, however, the
majority were in tree cavities (75.3%). These cavities (n 70)
were in southern rata (24.3%), kamahi (Weinmannia racemosa, 22.9%), broadleaf (Griselinia littoralis, 21.4%), totara
(7.1%), lancewood (Pseudopanax crassifolius, 1.4%), rimu
(1.4%), and tree fern (Dicksonia squarrosa, 1.4%). Dead trees
(species unidentified, 20.0%) composed the remainder of
cavity-nest sites.
The microclimates of tree-cavity nests varied less
than those of the ambient air and of unused tree cavities
(n 34 nests, Table 1). The main effect comparing differences
among sites was significant for most descriptive temperature
465
variables (Tmean, Tmax, Tmin, Trate) in both years (F 1.854–
19.584, P 0.001–0.034), except for Trange in 2007 (F 1.352,
P 0.187, Table 1). The significance of the main effect of
nest type differed by year and the five descriptive temperature variables, with nest cavities usually possessing superior
thermal qualities. Compared to unused tree cavities, nest cavities had more stable temperatures (smaller range), were more
insulated against the cold (higher minimum), and had lower
rates of hourly change (Table 1). Structurally, used tree cavities were similar to unused tree cavities, but vertical depth,
height of entrance hole, and the tree’s DBH differed, as unused cavities were deeper, typically lower to the ground, and
in trees with significantly smaller diameters (Table 2).
Only Tmin was associated with any of the three treecavity characteristics examined in our regression analysis of
nest cavities (n 18). Width of entrance hole was negatively
associated with minimum temperature (r2 0.33, F 7.98,
P 0.012), i.e., the minimum temperature was lower in cavities with wider entrances than in those with smaller entrances.
There were no other associations between any of the remaining descriptive temperature variables (Tmean, Tmax, Trange, Trate)
and the tree-cavity characteristics.
For our comparison of microclimates of tree cavities with
those of other cavity types, we located 45 first-clutch saddleback nests during the breeding season (October 2007–January
2008). During this study year a majority of birds nested in tree
cavities (60.0%), but nesting in other cavity types was common with 26.7% of pairs nesting in bank cavities and 13.3%
nesting in debris cavities. Because our 10 data loggers were
devoted primarily to recording tree-cavity microclimate,
however, we collected data from only a randomly selected
subset of other cavity types (n 8). We paired each of the
eight randomly selected non-tree-cavity nests with the nearest
tree-cavity nest and recorded temperatures simultaneously at
both locations. Our results indicate that within the 5-day periods of recording all the five temperature variables except rate
of change differed significantly; we found no significant differences, however, among cavity types or the interaction of
cavity type and day (Table 3). Neither did we find significant
thermal differences between the two categories of substrate,
as tree cavities and cavities in other substrates did not differ
significantly in any of the five descriptive temperature variables (Table 4).
DISCUSSION
Cavities appear to buffer eggs, nestlings, and attending adult
saddlebacks against oscillations in ambient temperature.
Tree-cavity temperature differed from ambient temperature,
with ambient temperature changing more and reaching lower
minima. These fluctuations in ambient temperature are potentially detrimental to reproduction, as ambient temperature usually does not reach levels suitable for development
of offspring (Webb 1987), especially in the cool, temperate
466
BRYAN RHODES
ET AL .
TABLE 2. Comparison of means o SE for nine characteristics of saddleback
nest cavities and unused cavities on Ulva Island, New Zealand, 2007–2008.
Sample sizes are uneven as not all measurements were made for each nest.
Characteristic and cavity type
Height of entrance (m)
Nest
Unused
Entrance hole width (cm)
Nest
Unused
Entrance hole length (cm)
Nest
Unused
Horizontal depth (cm)
Nest
Unused
Horizontal width (cm)
Nest
Unused
Vertical depth (cm)
Nest
Unused
Vertical height (cm)
Nest
Unused
Diameter at cavity height (cm)
Nest
Unused
Diameter at breast height (cm)
Nest
Unused
a
Mean
Range
n
2.11a o 0.19
1.23 o 0.11
0.61–6.50
0.27–2.70
42
35
14.39 o 1.47
18.70 o 2.62
6.70–52.50
6.50–75.10
35
36
31.95 o 2.79
27.50 o 3.95
7.90–74.00
7.40–96.10
35
36
23.66 o 1.60
22.29 o 1.93
9.50–55.10
10.00–71.00
35
36
21.63 o 1.74
20.94 o 2.09
10.40–59.40
6.90–75.20
34
35
14.04a o 2.02
34.41 o 7.18
0.00–43.00
0.00–200.00
34
35
40.97 o 3.66
43.94 o 5.59
2.00–100.00
1.60–154.00
34
34
160.41 o 12.22
157.95 o 7.17
64.00–406.00
80.00–246.00
33
34
178.43a o 18.65
109.05 o 13.36
99.10–406.00
156.00–174.00
18
15
Difference between nest and unused cavities significant at P 0.05.
climate of southern New Zealand (O’Donnell 2002). The vast
majority of birds use contact incubation to afford the proper
temperatures for development of their offspring and many
construct nests, which play a role in insulating eggs, nestlings,
and adults from ambient temperatures. Even if most nest types
insulate against ambient temperature, the protective nature of
and variability among tree cavities may allow rigorous selection of microclimate (Sedgeley 2001).
VARIABILITY IN TREE CAVITIES
It is not surprising that concealing a nest within a tree cavity
reduces heat loss over ambient conditions. Of greater importance is whether variability in tree cavities leads to a disparity
in cavity microclimate that birds are able to recognize, which
our data suggest. Structural variability seems to be common
among natural tree-cavity nests and may be critical in determining if cavities are used or remain unoccupied (Lõhmus
and Remm 2005, Remm et al. 2006). Tree cavities may occur
in abundance within forests but only small percentages are
suitable for nesting (Lõhmus and Remm 2005, Blakely et al.
2008). Structural characteristics likely are important in determining this suitability, as many properties of tree cavities
directly relate to factors such as cavity predation and microclimate. The ability to evaluate these factors may allow prospecting birds to assess the breeding potential of available cavities.
Numerous characteristics, ranging from the nest site to
landscape level, have been shown to influence the thermal
environment within tree cavities (Wiebe 2001, Radford and
Du Plessis 2003, Sedgeley 2006, Paclík and Weidinger 2007).
Trees with larger diameters, for example, are thought to mitigate thermal oscillations because they store heat better and
they possess thicker walls, which are more able to buffer internal and external temperature differences. We did not, however, test microclimate’s association with DBH because of
limitations of sample size and because measurements of DCH
indicated no significant differences in size between treecavity types. Others factors, such as greater exposure to solar
radiation, have also been shown to enhance microclimate in
tree-cavity nests, as trees are able to gather more thermal energy. The dimensions of tree cavities also are known to affect
internal temperatures because tree cavities with larger volumes retain greater quantities of heat (Paclik and Weidinger
2007), although we found that tree cavities with nests were
significantly smaller than unused tree cavities.
EFFECT OF NEST MICROCLIMATE ON THE SADDLEBACK
TABLE 3. Significance of fixed factors included within
our linear mixed models for each individual temperature
variable which examined differences between categories of cavity types (non-tree cavities; tree cavities) for
nesting saddlebacks on Ulva Island, New Zealand.
Variable and fixed factor
Mean
Day
Cavity
Cavity r day
Maximum
Day
Cavity
Cavity r day
Minimum
Day
Cavity
Cavity r day
Range
Day
Cavity
Cavity r day
Rate
Day
Cavity
Cavity r day
df
F
P
4.0
1.0
4.0
12.0
0.1
0.4
0.001
0.813
0.775
4.0
1.0
4.0
8.8
0.2
0.2
0.001
0.679
0.959
4.0
1.0
4.0
9.0
1.0
0.6
0.001
0.341
0.644
4.0
1.0
4.0
5.9
0.5
0.1
0.004
0.493
0.981
4.0
1.0
4.0
1.3
2.1
0.0
0.296
0.154
0.996
We did not examine characteristics beyond the site level;
however, we did find that the width of the entrance hole of
tree-cavity nests was negatively associated with minimum temperature, with minimum temperature explaining a
moderate proportion of our model. The association of treecavity-entrance width and temperature agrees with both Sedgeley (2001) and Paclík and Weidinger (2007) who found that
smaller entrances were better at maintaining internal temperature. This association may occur because wider entrances
allow more convection, which dissipates heat quicker to facilitate cooling of the cavity. Smaller entrances also may enhance
microclimate by limiting exposure to rainfall, which can create a negative thermal environment for nestlings similar to the
effects of lower temperatures through dampening (Radford
and Du Plessis 2003).
TABLE 4. Least-square means o SE and P values for
pairwise comparisons of tree-cavity (n 8) versus nontree-cavity nests (n 8) of the South Island Saddleback.
Means and P values were generated from repeatedmeasures linear mixed models with the five descriptive
temperature variables.
Variable
Mean
Maximum
Minimum
Range
Rate
Cavities
Non-cavities
P
13.2 o 0.46
14.8 o 0.80
11.5 o 0.72
3.3 o 0.60
0.2 o 0.04
13.0 o 0.46
14.3 o 0.80
12.2 o 0.72
2.7 o 0.60
0.2 o 0.04
0.813
0.679
0.341
0.493
0.469
467
We failed to detect any significant association of the other
nest cavity characteristics examined (horizontal width, DCH)
with microclimate although some of these characteristics have
been associated with microclimate in other studies (Hooge et
al. 1999, Wiebe 2001, Paclík and Weidinger 2007). In some
cases the nesting female’s body heat may be required to detect
microclimatic differences among cavities, as a higher gradient
would make any differences in insulative capacity more evident (Paclík and Weidinger 2007) and would also increase the
ability to detect those differences (but see Ardia et al. 2006).
Additionally, we recorded temperatures later in the breeding
season when temperatures were likely higher and less variable. Therefore, some associations may have been more pronounced earlier in the breeding season, when the benefits of
the insulative capacity of tree cavities would be more evident.
In addition, characteristics such as entrance size and height
of entrance from the forest floor are known to enhance the
quality of cavity nests in ways other than stabilizing microclimate. Higher cavities with smaller entrances, for example, are
known to reduce predation (Albano 1992, Hooge et al. 1999).
Our results demonstrate that tree cavities selected by
South Island Saddlebacks have more stable microclimates
than unused tree cavities in their territories. This study is one
of the first to demonstrate that a secondary-cavity-nesting bird
selects natural tree cavities with potentially greater microclimatic benefits than nearby unused tree cavities. Tree cavities
buffered thermal oscillations allowing saddlebacks to experience higher minimum temperatures and lower rates of temperature change, which both may minimize the demands of
thermoregulation on parents and offspring. The broader implications are, as we have seen in other studies (see Sedgeley
2001, Weibe 2001, Paclík and Weidinger 2007), that certain
structural aspects of tree cavities, particularly those associated with the entrance hole, may influence temperature and
cavity selection and may provide a beneficial nest microclimate for saddlebacks.
CAVITY NESTS NOT IN TREES
Although the majority of saddleback nests were located in tree
cavities, nests in other cavities (e.g., cavities in banks or vegetation debris) were still common. Tree-cavity nests and other
cavity nests we studied did not diverge in their thermal properties, as the substrate types did not differ in five related temperature variables. Exactly what aspects of cavities in banks
and vegetation debris allowed them to equal the thermal properties of tree cavities in this study is unknown, as high-quality
tree cavities are thought to provide better thermal characteristics (see above). Thermal properties of the cavity types may
have been similar because the cavity nests not in trees were
typically concealed within vegetation, which may reduce
the negative effects of moisture (increases thermoregulation
costs) on eggs and nestlings and may slow the rate of heat loss
caused by convection (Radford and Du Plessis 2003). Even
468
BRYAN RHODES
ET AL .
if the thermal properties of bank- and debris-cavity nests are
similar, such sites likely have other negative aspects (Camprodon et al. 2008). For example, they are typically lower than
tree cavities, if not in the ground, and so may be at greater risk
of nest predation (Joy 2000, Camprodon et al. 2008). Nonetheless, our data show that saddlebacks occasionally do nest in
these other cavity sites with comparable thermal properties,
suggesting that tree cavities may not be necessary to provide
saddlebacks with a thermal environment suitable for nesting.
GENERAL CONCLUSIONS
Given the abundance of literature devoted to cavity nesting,
the lack of investigations into the importance of microclimate
is surprising. Our study is one of the first to demonstrate that
secondary-cavity-nesting birds breeding in natural tree cavities select nest sites whose microclimate is less variable than
in other tree cavities available in their territories. To examine
the reproductive consequences of cavity microclimate, experimental manipulation may be necessary, as these birds may
be proficient in selecting natural nest cavities with suitable
microclimates and in the absence of experimental manipulation, variation in cavity microclimate may be too small for
any significant difference in reproductive success to be distinguished, especially if the number of nesting pairs examined
is low. Regardless, recent experimental studies have shown
increases in reproductive success or nestling quality with a
more beneficial (artificially induced) microclimate (Dawson
et al. 2005, Pérez et al. 2008). Therefore, selection of nest sites
with less microclimate variation may have long-term fitness
consequences for parents and their offspring and the effects
of microclimate on cavity-nesting birds may have been underestimated. Our results also suggest that other cavity types
(e.g., ground cavities), which for secondary-cavity-nesting
forest birds have been largely ignored, may provide nesting
sites suitable at least in terms of temperature regulation. Yet
the degree to which microclimate affects selection of a natural cavity remains obscure, as most likely a balance of factors contributes to the choice (Albano 1992, Fisher and Wiebe
2006, Remm et al. 2006). How selection for a less variable
microclimate is associated with other factors relating to cavity choice, such as protection from predators or intra- and interspecific competition (Remm et al. 2006, Aitken and Martin
2007), remains to be explored.
ACKNOWLEDGMENTS
We thank the Department of Zoology, University of Otago, and the
Department of Conservation (research contract no. 3575 to IGJ) for
providing funding for equipment and travel. We especially thank
Brent Bevan and Phred Dobbins and the Department of Conservation, Stewart Island, for providing logistic support during the field
season on Ulva Island. We also thank Lisa Hegg, Stephanie Hicks,
Sandra Hoeder, and several volunteers for assisting with nest finding. Thanks must also go to two anonymous reviewers for comments
that improved the quality of the manuscript.
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