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Habitat Use and Foraging Behavior of Eastern Bluebirds
(Sialia sialis) in Relation to Winter Weather
Todd J. Weinkam1, Gregg A. Janos1, and David R. Brown1,*
Abstract - Population declines of songbirds following severe winters draw attention to a
need to better understand behavioral responses to inclement weather. We used observations
of radio-tracked Sialia sialis (Eastern Bluebird) wintering in Kentucky to examine the effects
of weather on habitat use, group size, foraging behavior, and diet. Home ranges were
smaller than published estimates, and consisted of more open than wooded habitat, in proportion
to availability. Although habitat use appeared unchanged during inclement weather,
Bluebirds increased group sizes, and shifted from insectivory to frugivory during periods
of sub-freezing temperatures and snow cover. Fecal analysis confirmed the weather-driven
shift of diet. Inclement winter weather likely lowers the efficiency of insectivory, leading
to changes in social and foraging behaviors.
Introduction
Environmental factors and winter-specific resource constraints can affect the
behavior (Duriez et al. 2005), abundance (Meehan et al. 2004), and range limits
(Zuckerberg et al. 2011) of birds. Severe winter weather can reduce winter survival,
population size, and impact reproductive success in subsequent seasons (Porter et
al. 1983, Sauer and Droege 1990). Our ability to predict and interpret these processes
may be enhanced by a better understanding of winter-specific habitat and
resource requirements. Specifically, information about the interactions between
weather, habitat, and behavior may help explain how winter events influence population
patterns.
Sialia sialis L. (Eastern Bluebird, hereafter Bluebird) have been widely studied
during the breeding season (Gowaty and Plissner 2015), but much less is known
about their winter ecology. Reports of Bluebird mortality during harsh winter
weather (Pitts 1978, Wilson and Stamm 1960) followed by observable population
declines (Monroe 1978; Palmer-Ball 2015; Sauer and Droege 1990; Stamm 1979a,
b; Wilson 1962) demonstrate the potential for winter weather to affect their populations.
Recently, Wetzel and Krupa (2013) reported a positive correlation between
mean winter temperature and Bluebird abundance during subsequent breeding seasons
in central Kentucky, and suggested that the Bluebird population there may be
particularly susceptible to cold winters, in part because most birds are residents.
Although several investigators have examined Bluebird breeding habitat use
(e.g., Plissner and Gowaty 1995, Sloan and Carlson 1980, Stanback and Rockwell
2003), less is known about use of habitat during the winter. Allen and Sweeney
1Department of Biological Sciences, Eastern Kentucky University, Richmond, KY
40475.*Corresponding author - david.brown@eku.edu.
Manuscript Editor: Noah Perlut
Winter Ecology: Insights from Biology and Histor2y4(Special Issue X):XX–XX
2017 Northeastern Naturalist 24(Special Issue 7):B1–B18
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(1991) reported that winter home ranges of Bluebirds in South Carolina averaged
113.1 ha in size, an almost 10-fold increase from that of the breeding season. They
attributed this increase to greater winter energy requirements, and the need to search
outside their home range for food. Allen and Sweeney (1991) also found that wintering
Bluebirds used edge habitat more than expected based on availability.
Region-specific information on habitat requirements and animal behaviors is
important because population responses to environmental stimuli can vary across
a species’ range (Mehlman 1997, Whittingham et al. 2007). For instance, despite
a trend for increasing Bluebird abundance nationally over the past 4 decades,
Bluebird populations in central Kentucky have recently declined (Sauer et al.
2014 ). Mehlman (1997) demonstrated a trend for a decline in Bluebird abundance
following a series of severe winters in the 1970s, but this effect varied across the
species’ range.
Many wintering birds join flocks; larger groups allow individuals to spend
more time foraging as a result of decreased anti-predator vigilance by each individual,
and may enhance the efficiency of finding food (Morse 1970, Sridhar et al.
2009). Group size may increase during periods of higher energetic demands (Caraco
1979), such as those imposed by inclement winter weather. Factors that might
contribute to adjustments in group size by Bluebirds during unfavorable weather
have not been described.
Winters in temperate regions can create metabolically demanding conditions for
birds. Arthropods, an important food source for Bluebirds, are not always available
during winter, and Bluebirds include a wide variety of fruits in their winter
diet (Pinkowski 1977, Pitts 1979). Fruit is a critical component of the diet of many
other wintering neo-temperate bird species (Baird 1980), but its relative importance
for bluebirds is not clear. Snow is likely to affect availability of ground-active arthropods,
whereas ice, which can accumulate on tree branches during freezing-rain
events, may also impact fruit availability, and has been shown to disproportionately
affect the abundance of tree foraging bird species in subsequent years as compared
to open-habitat, ground-foraging species (Blais et al. 2001). If fruit is an important
food resource for Bluebirds in winter, identifying and describing conditions that
cause a shift to frugivory is important for better understanding the ability of Bluebirds
to survive such conditions.
Because unpredictable winter events are correlated with fluctuations of
Bluebird populations (Gowaty and Plissner 2015, Wetzel and Krupa 2013),
identifying the habitat requirements and factors potentially limiting populations is
important, especially within a regional context since Bluebird population changes
are spatially variable (Mehlman 1997, Sauer et al. 2014). The goals of this study
were to describe the habitat composition and size of the winter home ranges of
Bluebirds in the Bluegrass ecoregion of Kentucky, and to examine how weather
influences Bluebird habitat and space use, group size, group composition, foraging
decisions, and diet. We used radio-telemetry to locate individual Bluebirds
throughout the winter, and then related their behaviors to habitat availability and
weather conditions.
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Field-site Description
All field research was conducted at the US Department of Defense Blue
Grass Army Depot (BGAD), located in Madison County, KY (37°40'55"N,
84°13'16"W; Fig. 1). The installation consists of ~5907 ha, within which we focused
on a study area of ~850 ha containing open fields and pastures dissected
by woodlots, wooded stream corridors, roads, and buildings. The local population
of Bluebirds presumably included both residents and migrant Bluebirds that
breed at higher latitudes.
Figure 1. (A) Location of study area within the state of Kentucky. (B) Home ranges (95%
kernel isopleths) of Bluebirds within the Bluegrass Army Depot (white boundary line), and
(C) at a larger geographic scale within the study area.
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Methods
During January–February of 2010 and 2011, we located Bluebirds across the
study area and used 12 m x 2.5 m x 36 mm mist-nets to capture 1 or more birds
in each observed group. We used playback of Bluebird vocalizations to attract
birds into the mist-nets. We used elastic nylon-string harnesses (Rappole and Tipton
1991) to attach radio-transmitters (Holohil, BD-2; 0.9 g; average = 2.5 ± 0.2
[SD] % of body mass) to 19 Bluebirds (2010: n = 11 birds, 2011: n = 8 birds), and
uniquely marked each bird using 1 USGS numbered aluminum band and 3 color
bands. We determined the age of captured birds (second-year [SY] or after-secondyear
[ASY]) by examining the 10th primary coverts (Pitts 1985), and the sex of the
birds by using plumage characteristics (Pyle 1997). Each radio-tagged Bluebird
was tracked during daylight (08:00–18:00 hrs), typically 4–6 days per week, using
a Yagi 3-element antenna and Telonics TR-4 receiver (Telonics, Inc., Mesa,
AZ) for the duration of the transmitter battery life (60 days), or until transmitters
fell off, or birds were depredated. We used a homing method to locate birds and
visually confirmed their identity by sighting color bands or the transmitter’s whip
antenna. Upon locating a radio-tagged bird, we used a portable GPS/data management
device (Trimble Juno SB; Trimble Navigation Ltd., Sunnyvale, CA) to record
the individual’s geo-referenced location, date, and time. During each observation,
we recorded all data from a distance (typically >20 m) that appeared sufficient to
avoid influencing the behavior of individuals or flocks. The average (± SD) interval
between same-day observations was 81 ± 73 min.
Foraging observations were recorded whenever possible as part of radio-tracking
and began as soon as birds were visually located. Radio-telemetry allowed
us to locate birds from a distance (typically 25−100 m). Observations lasted 2−5
minutes and were conducted on all birds visible in the group. The average (± SD)
number of observations per group was 21 ± 5. Observations occurred throughout
the day, with an average (± SD) of 30 ± 11 per daylight hour. All Bluebirds in a
group typically foraged for the same type of prey. Foraging directed at arthropods
was treated as a single category because of difficulty identifying prey. We assumed
Bluebirds were foraging for arthropods when they exhibited their characteristic
drop-foraging behavior (ground sallying; Goldman 1975) and this assumption was
often confirmed by observation of arthropod ingestion. The absence of leaves from
most woody plants made instances of frugivory and type of fruit consumed quickly
discernible. Frugivory was recorded based on plant species, and we later combined
all such instances into a single category for analysis. Occasionally 2 different food
items were being consumed by Bluebirds in a group (e.g., frugivory interrupted by
sporadic drop-foraging by one or more group members), in which case we used the
dominant foraging mode to assign food type.
Individual Bluebirds were typically observed in conspecific groups in a relatively
small area. We estimated the size of the group and the sex of each group
member during observations of radio-tagged birds. Determination of group size
was often facilitated by extended flights of the entire group into open habitat, but
estimates made in forest habitat or when movements were minimal may be biased
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towards lower numbers. Thus, the estimates reported here likely represent minimum
group sizes.
We opportunistically collected fecal samples while banding birds and stored
them on filter paper. Dried fecal samples were scraped into a petri dish for sorting
and identification. Working under a dissecting microscope, we dripped small
amounts of ethyl alcohol (70%) into the dishes and separated seeds, arthropod
parts, and other materials using forceps and probes (Burger et. al. 1999). Arthropods
were sorted and identified to Order using references by Ralph et al. (1985),
Borror et al. (1989), and Burger et al. (1999), and counted based on the number
of heads, or pairs of mandibles, wings, elytra, chelicerae, or other distinguishable
body segments. For example, a head capsule was counted as one individual and
every two mandibles of particular taxa were counted as one individual. We sorted
by species and counted all seeds found in the samples. To identify seeds, we used
reference material including books (Jones 2005), the seed collection of the EKU
Herbarium, and dissected field collections from the study area. We also consulted
with an entomologist (A. Braccia, Eastern Kentucky University, Richmond, KY,
pers. comm.) and a botanist (R. Jones, Eastern Kentucky University, Richmond,
KY, pers. comm.) for verification.
All GPS locations were downloaded into ArcMap version 10.0 (ESRI, Redlands,
CA) and projected using the NAD 1983 state plane (feet) Kentucky coordinate
system. We generated home-range estimates using kernel density estimation (KDE)
techniques that produce utilization distributions (UD) based on the relative density
of telemetry locations over an area. We used the Geospatial Modelling Environment
(Beyer 2012), a Program-R based supplement to ArcGIS, to conduct KDE
analyses for all birds with >20 locations (n = 9). Results of KDE analysis are sensitive
to both the resolution of the evaluation area (grid size) and the bandwidth
(Seaman and Powell 1996). We used a fixed-bandwidth value of 20,000 for all
analysis because it provided a suitable balance between over- and under-estimation
of home ranges. We depicted a home-range boundary as the 95% isopleth of the
kernel probability density function (Seaman et al. 1999). The core home-range
area, representing areas of intensive use, is defined here by a 50% isopleth of the
kernel probability function.
The nonselective capture of Bluebirds used in our sampling design excluded
many individuals present in the study area, so comprehensive insight of Bluebird
home range overlap across the study area was not possible. However, 5 birds in
2010 appeared to occupy distinct, yet adjacent, home ranges. We calculated the
percentage of overlapping area between these adjacent home ranges using ArcGIS.
We used satellite imagery and a maximum-likelihood supervised image-classification
process to categorize habitats (Palmeirim 1988), employing the image
classification tool in ArcMap 10.0 to designate each pixel of the 1-m–resolution
RGB satellite image of the study area (US Department of Agriculture, 2006) as
either (1) wooded or (2) open habitat types based on the color profiles of the
pixels. Representative training samples of the satellite imagery were selected
using unambiguous areas of land cover within the study area and then further
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adjusted to minimize overlap within resultant scatterplots of the red, green, and
blue visual spectra for the 2 habitat category profiles. Based on the color profiles
of these training samples, we assigned all pixels in the study area to either open
or wooded categories. Further correction of the resulting raster image map (e.g.,
removal of artifacts such as improperly categorized shadows) was completed
manually using Adobe Photoshop CS (version 5.1, Adobe™) and referencing the
original satellite imagery.
Weather data were collected from an on-site weather monitoring station maintained
by the US Army. Air temperature and wind speed were recorded every 15
min; thus, no weather observations were more than 7 min removed from an instantaneous
reading. Snow depth was recorded daily 53 km north at the Lexington
Bluegrass Airport (38°2' 23.99"N, 84°36'35.99"W) and collected from the National
Oceanic and Atmospheric Administration’s National Climatic Data Center (www.
ncdc.noaa.gov).
Statistical analysis
We quantified descriptive Bluebird 95% and 50% home-range metrics including
area, habitat composition, and overlap between adjacent home ranges with ArcGIS.
We compared the habitat composition of home ranges to that of all unused habitat in
the study area using a one-sample t-test. In this case, the study area was defined as
a polygon bound by the most-outward vertices of all bird home ranges over 2 years.
We quantified the habitat surrounding each bird location within a 5-m radius to determine
habitat use at each location. These data were used to compare habitat use
both when snow cover was and was not present (i.e., ≥2.54 and 0 cm, respectively,
as recorded from the Lexington Bluegrass airport) using a Mann-Whitney U-test.
We used a nested-ANOVA to investigate the potential effect of snow cover
on the distance of birds to edge habitat (the boundary between open and wooded
spaces). For this analysis, we were interested in whether Bluebirds preferentially
used edge habitats during periods of snow cover. To account for the repeated observations
of individuals, each Bluebird location was nested by the identity of its
group as determined by the individual radio-tagged bird that was used to find the
group. Thus, group identity is based on the radio-tagged individual. We also used
a nested-ANOVA to examine the potential effect of freezing temperatures (i.e.,
≤0 °C) on Bluebird group size, again with each observation nested within group
identity, as based on individual radio-tagged birds. Similarly, group sizes were
compared during periods of snow cover (≥2.54 cm) and when snow was absent (0
cm) using a nested-ANOVA, with each observation of group size nested within a
flock represented by the radio-tagged bird. We used a paired samples t-test to compare
the number of males and females in groups. Sex composition of groups was
also compared between periods of snow cover and when snow was absent using a
nested-ANOVA, with the count of each sex nested within the flock as represented
by the radio-tagged bird. Observations were not included in these analyses when
the sex of one or more group members was not determined.
To test for influence of snow cover, air temperature, and wind speed on the
foraging behavior of Bluebirds, we used a logistic regression analysis. For this
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analysis, foraging behavior, the response variable, was classified dichotomously
as directed towards fruit or arthropod prey. Group identity, as determined by the
presence of a radio-tagged individual, was included as a random effect to account
for repeated sampling.
For diet, we report the percent occurrence of taxa (i.e., the percent of samples in
which a taxa occurs; Rosenberg et. al. 1990). To test for differences in diet based on
weather, we used a 2 x 2 contingency table analysis to compare the relative occurrence
of fecal samples with and without seeds when the temperatures were greater
or less than 0 °C.
All analyses were performed using SPSS v. 18.0 (SPSS Statistics 2009). All
means are reported ± SE, unless otherwise noted. Statistical significance was accepted
at α = 0.05.
Results
There were 21 and 19 days of snow cover of ≥2.54 cm during January–February
of 2010 and 2011, respectively (Fig. 2). Daytime temperatures averaged 31.2 °C
and 31.5 °C during January–February of 2010 and 2011, respectively.
We radio-tagged and tracked 19 individuals, including 15 males (7 SY and 8
ASY) and 4 females (2 SY and 2 ASY), for an average span of 23 ± 15 (SD) days
(range = 1–51). We obtained an average of 20 ± 13 (SD) locations per individual
(range = 2–44). Each day of tracking yielded an average of 1.7 ± 0.6 (SD) locations
per individual.
Twelve of the 19 radio-transmitters were subsequently recovered either on the
ground or among foliage. Whereas most of the recovered transmitters appeared to
have fallen off (the birds were later re-sighted), at least 3 Bluebirds with transmitters
appeared to have been killed by predators as indicated by a transmitter located
among or near numerous feathers or other body parts.
In 2 cases, location data of 2 Bluebirds were combined for analyses. One pair
of males from the same group was captured and radio-tagged simultaneously;
87% of 39 locations were shared by both birds (i.e., the two birds were typically
found moving or foraging together). The other 2 birds (a male and female)
were also captured simultaneously, but were tracked over separate time periods;
the female was tracked for 14 days, after which the radio-tag fell off. We then
recaptured and radio-tagged the male at the same location, and tracked it for 11
days. An exploratory home-range analysis for the 2 birds revealed nearly identical
overall and core home ranges, so we combined their locations into a single
home-range analysis.
Including the 2 shared home ranges described above, 9 Bluebirds (or pairs) had
>20 location observations (average = 30.3 ± 7.5 [SD], range = 22–44). For these
9 birds, the average home-range area (95% utilization distribution) was 29.2 ± 2.4
ha (range = 16.3–42.3 ha), and the average core home-range area (50% utilization
distribution) was 7.1 ± 0.6 ha (range = 3.8–9.9 ha) (Fig. 1). Of 5 birds that had an
adjacent home range, an average of 9.4 ± 2.5% of the 95% home-range–estimate
area was shared by neighboring birds. Core home ranges did not overlap.
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Bluebird home ranges consisted of 39.6 ± 2.6% wooded habitat and 60.4 ± 2.6%
open habitat. The percentage of wooded habitat within home ranges did not differ
significantly from the overall percentage of wooded habitat in the entire study area
(35.7%) (t8 = 1.5, P = 0.18). Similarly, core home-range areas consisted of 41.1 ±
3.5% wooded habitat and 58.9 ± 3.5% open habitat, which was not significantly different
from the composition of available habitat in the study area (t8 = 1.6, P = 0.16).
The mean percentage of wooded area surrounding each observation (5-m radius)
was 60.0 ± 2.0% (n = 344). Habitat within the 5-m radius around each point was
frequently either entirely (100%) wooded or entirely open (0% wooded). The mean
percentage of wooded habitat per observation (5-m radius) during periods of snow
cover (54.4 ± 4.3%, n = 94) did not differ from that during periods when snow was
Figure 2. Snow depth at Lexington Bluegrass Airport during January–February of 2010
and 2011.
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absent (63.1 ± 2.5%, n = 250) (Mann-Whitney U-test: P = 0.14). The distance of
Bluebirds to the edge of wooded habitat did not differ during periods of snow cover
(F1,91 = 0.01, P = 0.92).
Radio-tagged Bluebirds were almost always found in conspecific groups (97%
of observations). Average group size was 5 ± 0.1 individuals (n = 300), with a
maximum of 16 individuals observed in a group that included a radio-tagged bird
(Fig. 3). Groups included more males (2 ± 0.1) than females (1 ± 0.1) (t272 = 20.0,
P < 0.001), and the presence of snow cover did not influence the sex composition
of groups (F1,83 = 2.6, P = 0.11). Mean Bluebird group size during below-freezing
temperatures (5 ± 0.2 individuals) was larger than when temperatures were above 0
°C (4 ± 0.2 individuals) (F1,62 = 4.8, P = 0.032). Similarly, group size during periods
of snow cover (6 ± 0.3 individuals) was larger than that when snow was absent (5
± 0.1 individuals) (F1,266 = 5.0, P = 0.02).
Flocks were typically found with all members foraging on either arthropods or
fruit (91% of n = 235 observations), but not both. Most foraging attempts were directed
at arthropods (>65% of observations for either year), and relative frequencies
of frugivory for different fruits varied between years, with Phoradendron leucarpum
Raf. (Oak Mistletoe) most common in 2010, and Celtis occidentalis L. (Hackberry)
the most common in 2011 (Table 1). Logistic regression analysis indicated that frugivory
was more likely than foraging on arthropods during observations with low
temperatures, low wind speed, and the presence of snow (Table 2).
Analysis of fecal samples from 33 birds included 105 items identified from 7
arthropod orders (Mesostigmata, Araneae, Orthoptera, Hemiptera, Hymenoptera,
Figure 3. Eastern Bluebird group sizes (n = 300) during January–February of 2010 and 2011.
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Coleoptera, and Lepidoptera) and 5 species of seeds (Table 1). Seeds represented
60% of all items and occurred in 22 samples. There was an equal frequency of fecal
samples that contained only arthropods (33%), only fruit seeds (33%), and both arthropods
and fruit seeds (33%). Fecal samples collected when the temperature was
below 0 °C (n = 20) were 4.4 times more likely to have seeds present than samples
collected at higher temperatures (n = 13) (χ2 = 4.06, P = 0.04, Fig. 4).
Table 2. Results of logistic regression analysis on forage type (fruit or arthropods) by weather conditions.
Low air temperature, low wind speed, and snow presence increased the likelihood that eastern
bluebirds foraged on fruit. Group identity was included as random variable to account for repeated
measurements so coefficients and odds ratio are not reported.
95% CI
Wald’s P-value B Lower Upper Odds ratio
Low air temperature 14.5 less than 0.001 0.19 0.09 0.29 1.22
Low wind speed 8.2 0.004 0.57 0.18 0.96 1.77
Snow presence 5.3 0.020 1.23 0.18 2.28 3.44
Group identity 13.0 0.791 - - - -
Table 1. Percent foraging observations of Eastern Bluebirds in 2010 (n = 139), 2011 (n = 96), and in
both years combined among different food categories. Other fruit included Prunus serotine Ehrhart
(Black Cherry), Symphoricarpos orbiculatus Moench( Coralberry), Lonicera japonica Thunb. (Japanese
Honeysuckle)*, and Toxicodendron radicans L. (Poison Ivy)*. Plant species with an asterisk
were also detected in fecal samples.
Food category Both years (%) 2010 (%) 2011 (%)
Arthropods 71.9 76.3 65.6
Hackberry* 10.2 0.1 24.0
Oak Mistletoe* 8.5 13.7 1.0
Red Cedar* 6.0 7.9 3.1
Other fruit 3.4 1.4 6.3
Figure 4. Percent of fecal
samples with seeds present
(i.e., only seeds, or seeds and
arthropods) or with only arthropods
collected from Eastern
Bluebirds during January–
February of 2010 and
2011 when temperatures were
above or below freezing.
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Discussion
Home range and habitat use
The mean estimated 95% winter home range size of Bluebirds in this study (29.2
ha) was smaller than that reported by Allen and Sweeney (1991) in South Carolina
(113.1 ha). However, this difference is likely at least partially due to the use of different
home-range estimation techniques; the minimum convex polygon method
used by Allen and Sweeney (1991) is known to overestimate home-range sizes (Anderson
1982), whereas the kernel-based utilization distribution that we used emphasizes
focal areas of intensive use and minimizes the influence of distant, isolated
locations (i.e., outliers). The mean size of 95% winter home ranges of Bluebirds in
this study was similar to that of Bluebirds during seasonal transition periods (i.e.,
August–November and February–April) described by Savereno (1991) in South
Carolina (28.0 ha). In his study, home-range area was calculated using cumulative
area curves (Odum and Kuenzler 1955), which is more comparable to the kernelbased
utilization distribution used in this study than the minimum convex polygon
method. Our sample size was too small to examine the possible effects of sex and
age on the size of Bluebird winter home ranges.
With the exception of 2 pairs whose space use data were combined for
home-range analysis, we found little (95% home-range estimates) and no (core
home-range estimates) overlap in the home ranges of Bluebirds. Territory maintenance
and defense during the non-breeding season has been documented in
resident and migratory bird species, and is typically driven by variation in the
abundance of limited food resources (Brown et al. 2000, Safina and Utter 1989,
Townsend et al. 2010). The distinct, minimally overlapping home ranges of Bluebirds
in this study may also represent resource-influenced territoriality, but, in
this case, the territories appear to be specific to groups. The 2 pairs that occupied
almost identical home ranges over 45 days of radio-tracking suggest that Bluebird
groups (or certain pairs) persist for extended periods of the winter , but the specific
roles of these relationships and other potential effects of group behavior on
space use of wintering Bluebirds have yet to be determined. It is also possible that
winter territoriality is related to defense of breeding territories. Thomas (1946)
noted that wintering Bluebirds in Arkansas sometimes engaged in intraspecific
competition (i.e., fighting and singing) around nest boxes, suggesting that defense
of nest sites may occur throughout the year, though to a lesser degree in winter.
Thus, in some populations, the co-occurrence of a male and female within home
ranges may be related to the winter maintenance of a breeding territory.
Home ranges of Bluebirds in this study were composed of more open habitat
than wooded habitat, although wooded areas made up almost 40% of home ranges.
Habitat structure was not measured in this study, but, in all cases, home ranges
included a heterogeneous mixture of both open and wooded areas, including lone
trees and wooded corridors. We found no difference in habitat composition between
Bluebird home ranges and the study area outside of those home ranges, and also
no difference in habitat composition between core (50%) and 95% home ranges.
Because we used radio-telemetry to locate birds, our observations should not be
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biased by differences in detectability between habitats. Allen and Sweeney (1990)
and Savereno (1991) reported that wintering Bluebirds in South Carolina used edge
habitat more than expected, and Levey et al. (2008), also in South Carolina, found
that movement along edges was preferred by wintering Bluebirds over using open
space within corridors. Because we only categorized open and wooded habitat, our
data cannot test if Bluebirds prefer edge habitat, as has been found in other studies
(Allen and Sweeney 1990, Levey et al. 2008, Savereno 1991).
Mean distance to edge did not change during periods of snow cover, nor did
the mean percentage of wooded habitat within a 5-m radius. These results suggest
that Bluebirds do not dramatically shift habitat occupancy during periods
of snow cover, and that their presumed preference for edge habitat is unaffected.
Petit (1989) demonstrated that wintering woodland birds move into habitat patches
with greater cover (i.e., mature pine stands) during harsh weather, but habitat occupancy
by Bluebirds in this study appeared to be unchanged by snow cover of
at least 2.54 cm in depth. Brotons (1997) described how Parus ater L. (Coal Tit)
responded to snow presence by changing foraging methods within a patch rather
than seeking a new habitat, and our results suggest that Bluebirds may respond to
the presence of snow in a similar way. For example, Pinkowski (1977) described a
positive relationship between Bluebird foraging height and temperature as well as
sunshine percentage (i.e., the proportion of time that shadows were cast), possibly
as a response to changing insect detectability related to weather. In that scenario,
a change of habitat as temperatures drop may be unnecessary to meet immediate
foraging needs, requiring only a behavioral response within the habitat. Even a shift
toward frugivory may not require movement to a new habitat (e.g., into a wooded
area) if fruits are available nearby. Despite no apparent change in habitat occupancy
during snow cover, Bluebirds may cope with unfavorable weather and its
associated energetic demands in other ways, such as by shifting diet or group size.
Several studies have documented communal night roosts in trees and bird houses
during cold weather and snowstorms (Forbush 1929, Frazier and Nolan 1959, Pitts
1978, Thomas 1946), suggesting weather influences bluebird habitat use and group
size during nighttime. Because of site access limitations during the night, we were
unable to include such behavior as part of this study, although we did observe a
daytime roost in a tree of 19 individuals during a severe winter storm.
Foraging behavior
Bluebirds in this study were more likely to engage in frugivory when temperatures
and wind speeds were low, and when snow cover was present, with snow
cover having the strongest effect. Low temperatures decrease arthropod activity
(Mellanby 1939), and snow reduces insect availability to Bluebirds (Frazier and
Nolan 1959, Pitts 1978), particularly by covering ground-active arthropods. Since
drop foraging is the most common foraging method used by Bluebirds in all seasons
(Gowaty and Plissner 2015), it’s not surprising that snow cover causes a shift
to alternative methods. Together, low temperatures and snow cover may make insectivory
less reliable and possibly increase the cost of this strategy to a suboptimal
level. Increased frugivory when wind speeds were low may indicate an ability of
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2017 Vol. 24, Special Issue 7
Bluebirds to manage thermal stress by altering foraging behavior. Grubb (1975)
found that several species of woodland birds foraged at lower heights during periods
of high wind (i.e., 2–3 m/s), and suggested that birds seek areas of low thermal
stress (i.e., closer to the ground) under windy conditions. Similarly, birds have been
shown to avoid windward edges of habitat patches and favor leeward edges (Dolby
and Grubb 1999). High winds may encourage insectivory and discourage frugivory
as a way to minimize thermal loss to wind, which would likely increase when Bluebirds
forage for fruits high in trees. Both Oak Mistletoe and Hackberry fruit, the 2
most common targets of frugivory, were typically located well above ground level.
The presence of high wind may limit the ability of Bluebirds to forage efficiently
during low temperatures and snow cover; so, when combined, these factors may
create the most energetically demanding (and behaviorally restrictive) conditions
that Bluebirds experience during winter.
The fruits most frequently consumed by Bluebirds in this study differed between
years. For example, based on observational data we found that Oak Mistletoe frugivory
accounted for 13.7% of all foraging observations in 2010, and only 1% in
2011. Instances of Hackberry frugivory displayed an opposite pattern with 0.07%
and 24.0% of all observations in 2010 and 2011, respectively. Although both are native
species, Oak Mistletoe tends to have relatively high lipid content (Stiles 1993),
whereas Hackberry has low levels of lipids (Johnson et al. 1985, Stiles 1980).
The availability of food was not determined in this study, so it is not possible to
determine whether the relative abundances of these food items accounted for this
pattern. Nonetheless, the composition of fruit in the diet of wintering Bluebirds
occupying the same general area can differ considerably between years. Therefore,
the relative value of any one fruit resource over another may vary annually, just
as the relative value of fruit over arthropods may depend on immediate weather
circumstances. Although Bluebirds are known to consume at least 60 types of fruit
in winter (Hoyo et al. 1992). Most winter studies of Bluebirds in temperate North
America document the consumption of fewer than 10 fruit species, most of which
represent only a minor dietary component (Morland 1978, Pinkowski 1977, Pitts
1978, Savereno, 1991). We observed Bluebirds foraging on 7 types of fruit, but
most foraging was directed toward fruit of Hackberry, Mistletoe, and Juniperus
virginiana L. (Red Cedar). Although fruits have been reported as important for
wintering Bluebirds elsewhere, these 3 species were absent or only a minor dietary
component for wintering Bluebirds in Ohio (Morland 1978), South Carolina (Savereno
1991), and Tennessee (Pitts 1979), suggesting regional or temporal differences,
likely based on availability.
Arthropod segments, comprising at least 7 different orders, were the most common
items in fecal samples, followed in decreasing order of occurrence by seeds
of Red Cedar, Oak Mistletoe, Poison Ivy, and Japanese Honeysuckle. Seeds were
more commonly found in fecal samples collected during periods with sub-freezing
temperatures reinforcing our conclusions from foraging observations that Bluebird
diets shift toward fruit during periods with low temperatures.
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Group size
If the metabolic costs of birds increase during unfavorable weather conditions
(i.e., low temperature), individuals may have to spend more time foraging to meet
metabolic demands. One strategy for increasing an individual’s foraging time is
termed the “group-size effect”, whereby individual foraging time can increase with
flock size as a result of improved collective anti-predator vigilance (Beauchamp
1998, Caraco 1979). Hogstad (1988) observed this effect among flocks of wintering
Poecile montanus von Baldenstein (Willow Tit) during cold days. Large Bluebird
flocks during snowstorms have been sporadically reported elsewhere during winter
(e.g., Thomas 1946), including accounts of Sialia currucoides Bechstein (Mountain
Bluebird) and Sialia Mexicana Swainson (Western Bluebird) (Allen and Brewster
1883, Henderson 1903). By assembling in larger numbers during inclement
weather, Bluebirds may be able to locate new food sources while simultaneously
benefiting from a group-size effect. Because Bluebird group size was larger during
periods with snow cover, when frugivory was also higher, increased flocking
behavior as a strategy to locate fruit resources seems possible (e.g., Elgar and Catterall
1982, Ficken 1981).
Thomas (1946) described “roaming and shifting” among Bluebird flocks during
winter, suggesting a lack of cohesiveness within Bluebird groups. Although accurately
estimating group membership was not always possible, it was not uncommon
to repeatedly observe the same uniquely banded individuals together over a period
of weeks. The 2 pairs of Bluebirds whose home-range estimates showed considerable
overlap is an additional indication that group cohesiveness, as well as stable
territoriality, can be maintained during the winter months. So it appears that Bluebirds
maintain winter territories in small groups through most of the winter, but
group membership may change, particularly during periods of inclement weather
when group size increases. More work is needed to understand how group size and
membership is related to foraging.
Our results suggest Bluebirds respond to inclement winter weather by changing
foraging behaviors and increasing group size; however, it remains to be determined
how these responses are driven by the distribution of food, their ability to find resources,
predator avoidance, and even thermoregulation (e.g., communal roosting).
Although we detected no mortality as a result of winter weather, our sample size to
do so was small. Winter weather may lead to mortality by reducing food availability
and contributing to physiological stress through heat loss. Concurrent changes in
foraging and space-use behavior may also make birds more vulnerable to predation.
We found evidence for changes in behavior in response to weather, but more work
is needed to understand the population-level impacts.
Acknowledgments
We thank A. Colwell and N. White of the Bluegrass Army Depot for providing logistical
support and access to the site. We’re grateful for taxonomic consultations with A. Braccia
and R. Jones. G. Ritchison, and C. Elliott provided comments on an earlier version of
the manuscript. S. Smedley, T. Wickman, and 2 anonymous reviewers provided valuable
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2017 Vol. 24, Special Issue 7
comments on the manuscript. This work was supported by a grant from Eastern Kentucky
University. Procedures related to the capture and handling of Bluebirds were reviewed by
Eastern Kentucky University’s Institutional Animal Care and Use Committee and approved
as Protocol #01-2010.
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