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22001188 SOUTHEASTERN NATURALIST V1o7l(.1 1):79,5 N–1o0. 31
The Influence of Temperature on Black-Capped Vireo
Nest-site Selection
Ronnisha S. Holden1, Michael L. Morrison1,*, and Heather A. Mathewson2, 3
Abstract - Reproductive success is highly influenced by nest-site selection for avian species
in breeding habitats, and variation in the physical environment can drive small-scale
changes in the nest-site selection process. We examined the influence of temperature on
Vireo atricapilla (Black-capped Vireo; hereafter Vireo) nest-site selection at Kerr Wildlife
Management Area (KWMA) in Kerr County, TX (March–July 2013 and 2014). We
measured ambient temperature across points that represented the continuum of vegetation
characteristics used by Vireos at our study sites during the breeding season. We also found
and monitored 181 Vireo nests, collected vegetation data, and compared vegetation characteristics
between areas used and not used by Vireos. Finally, we investigated whether
Vireo nest-site characteristics changed over the course of the breeding season in relation to
the temperature profile of vegetation at our study sites. As expected, temperature increased
over the course of the breeding season. Vireo nest sites had higher percent shrub cover than
areas not used for nesting by Vireos. Vireos selected different vegetation characteristics
for nesting as the breeding season progressed, but we did not find differences in temperature
across vegetation types, suggesting that temperature is not the driving factor in Vireo
nest-site selection in locations where temperatures remain consistently high throughout the
breeding season. However, we could not directly measure temperature at nest-site locations.
Therefore, Vireos may exhibit some degree of thermal preference at smaller spatial scales.
Additionally, our results suggest that Vireos may require nesting habitat with more shrub
cover than previously recommended.
Introduction
Factors including microclimate, food availability, and vegetation composition
and structure within an organism’s breeding habitat can influence the survival of
adults and young, and ultimately, impact persistence of the species. For avian species
in breeding habitats, reproductive success is highly influenced by nest-site selection,
and variation in the physical environment can drive small-scale changes that influence
the nest-site selection process. Thus, changes in the physical environment, such
as elevated seasonal and climatic temperatures, can influence patterns of habitat use
for some species (Barnagaud et al. 2013, Cox et al. 2013a, Martin et al. 2015).
Guthery’s (1997) useable-space hypothesis (i.e., based on an area that physically
and physiologically supports the adaptations of a species) combines principles of
habitat selection and use, while considering changes in environmental constraints,
to determine the influence of these factors on animal behavior and reproductive
1Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station,
TX 77843. 2Natural Resources Institute, Texas A&M University, College Station, TX
77843. 3Current address - Wildlife, Sustainability, and Ecosystem Sciences, Tarleton State
University, Stephenville, TX 76402. *Corresponding author - mlmorrison@tamu.edu.
Manuscript Editor: Paul Leberg
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success over time. Changes in temperature can act as an environmental constraint
that can reduce the availability and potential quality of habitat space (Forrester et
al. 1998, Tieleman et al. 2008). This potential loss of useable habitat space is an
important concept for the management of species of concern.
Evidence suggests that temperature variation may contribute to habitat loss and
degradation; thus, it is imperative that we consider the influence of temperature—
both locally within a season and across longer time spans (i.e., climatic change)—
on threatened and endangered bird species (Huntley et al. 2006). Vireo atricapilla
Woodhouse (Black-capped Vireo; hereafter, Vireo) is a federally endangered neotropical
migratory songbird threatened by habitat loss and other anthropogenic
factors. Vireos breed in central Oklahoma, central and southwest Texas, and northeastern
Mexico (Grzybowski 1995).
In Texas, the Vireo breeds from April through July in shrubland vegetation,
including deciduous and non-deciduous woody cover, comprised of Quercus
virginiana Mill (Live Oak), Quercus havardii Rydb. (Sand Shinnery Oak), and Juniperus
asheii J. Buchholz (Ashe Juniper) mottes (Graber 1961). Vireos nest from
0.2 m to 3.0 m (usually 0.5 m to 2.0 m) above the ground (Graber 1961, Grzybowski
1995) with ~30‒50% cover around the nest (Grzybowski 1995), although variation
exists among different areas within the breeding range (Grzybowski et al. 1994;
Pope et al. 2013a, b; Wilkins et al. 2006).
Previous studies suggest Vireos have a preference for specific nest heights (Grzybowski
1995) and amount of concealment around the nest (Pope et al. 2013a, b).
Colon et al. (2017) found that Vireo nest placement changed in relation to drought
conditions, with Vireos using the evergreen Ashe Juniper as a nest substrate more
often when conditions were dry and deciduous substrates under moderate conditions.
Our goal was to explore how temperature may influence nest-site selection of
the Black-capped Vireo in central Texas across the breeding season.
Study Area
We conducted our study at the Kerr Wildlife Management Area (KWMA),
located in the Edwards Plateau ecoregion of central Texas, which consists of savanna
grassland, oak shrubland, oak–juniper woodland, and deciduous woodland.
Vegetation management at KWMA includes prescribed burning, slash and burn,
bulldozing, understory thinning, and occasional cattle grazing. Vireos at KWMA
inhabit mid-successional Live Oak and Sand Shinnery Oak mottes, and oak–juniper
woodlands of varying densities and heights (Pope et al. 2013a, b). The habitatmanagement
activities at KWMA maintain suitable vegetation for Vireos, and land
managers conduct extensive Molothrus ater (Boddaert) (Brown-headed Cowbird)
trapping efforts to minimize the negative effect of brood parasitism on nesting Vireos
and other breeding birds.
Study-site selection
We chose study sites at KWMA based on known occurrence of Vireos and specifically
sought areas with variation in vegetation height, canopy cover, and shrub
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cover (Graber 1961, Pope et al. 2013b). In 2013, we selected a 59-ha study site
on the west side of KWMA that was last burned in 1999. Personal observations
(R.S. Holden) prior to data collection indicated that vegetation on the study site
was variable, but that overall, shrubs were taller and canopy cover was higher
when compared to other areas of known Vireo occupancy at KWMA. In 2014, we
selected 2 study sites: one 22-ha study site last burned in 2007 that had substantial
woodland and little grassland cover; and one 60-ha study site last burned in 2011
that consisted of shorter vegetation than the 2013 study site. We selected different
sites between years because we were interested in the overall adaptive responses of
Vireos to changes in temperature and vegetation, and not differences between sites.
Methods
Ambient temperature
We recorded temperature across each study site using Lascar EL-USB temperature-
data loggers (Lascar Electronics, Erie, PA). We created a grid network
of points using ArcGIS at 200-m spacing (200 m x 200 m grid) within which we
systematically placed the temperature-data loggers at each grid point. This spacing
allowed complete coverage of each study site. We attached the data loggers to
1-m wooden stakes and covered them with green plastic cups to protect the loggers
from direct sunlight. If a point location along the grid was bare ground or only covered
in a herbaceous ground layer, we placed the data logger in a random position
within the nearest clump of vegetation to better represent potential Vireo nesting
and foraging substrates and to avoid temperature spikes from direct sunlight;
random-point locations were no more than 10 m away from the original location.
The data loggers recorded temperature in degrees Celsius (°C) every hour between
late March and late July during the 2013 and 2014 Vireo breeding seasons.
Site vegetation
Using the grid network of points created for temperature-data logger placement,
we measured vegetation across each site once during the Vireo breeding season to
quantify vegetation characteristics within areas that were used and not used by Vireos
for nesting (see below). At each grid point, and at 4 points located 5 m from the
grid point in each cardinal direction, we recorded canopy cover to the nearest 10%
using a tubular densitometer. We also used a 3-m range pole to measure height of
the vegetation at each of these 5 points. We established a 5-m–radius circle around
the center point and divided the circle into 4 quadrants based on the 4 cardinal
directions. Within each quadrant, we visually estimated the percent woody-shrub
cover to the nearest 10% for shrubs less than 2 m tall.
Territory establishment and monitoring
To locate Vireos on each study site, we again used the grid network to conduct
transect surveys. We walked at a ~1-km/hr pace from point to point from sunrise to
13:00 to detect singing male Vireos, and marked their locations with a Garmin Rino
GPS unit. We surveyed each transect 3 times between early March and late April,
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and used the GPS point locations of Vireo detections to relocate birds for subsequent
monitoring. We revisited each detection location at 3‒5-d intervals to map
territories, and considered males territorial if we detected them within a specific
location for ≥4 weeks. These surveys were not designed to estimate abundance, but
rather as a focus for nest searching (see below).
Nest searching
We conducted nest searching within Vireo territories (see above) from late
March to late July in 2013 and 2014. We used behavioral cues of the birds (e.g.,
alarm calls, food carries, territorial behavior) to locate nest sites. We monitored
nests every 2‒3 d and determined nest fate (i.e., nest failed or fledged). After a nest
failed or fledged, we continued to monitor and search the territory, every 3‒5 d,
looking for subsequent nesting attempts. Data on nest success are not presented in
this paper.
Nest vegetation
To identify the vegetation characteristics of each active nest, we recorded measurements
at all nests within monitored territories in which we found at least 1
Vireo or Brown-headed Cowbird egg or young. We recorded nest height, canopy
height, percent shrub-cover, and percent canopy-cover only after all nests had
fledged or failed at the end of the season. Nest-vegetation measurement methods
followed the procedures used for the above-mentioned vegetation measurements at
temperature-data logger locations.
Data analysis
Ambient temperature. We calculated the mean temperature, maximum temperature
(i.e., highest temperature recorded at each data-logger point location), and
the average maximum temperature (i.e., mean of daily maximum temperatures on
each data logger) for all temperatures recorded at site-vegetation locations for each
temperature-data logger.
Vegetation. We obtained average values for the vegetation-metrics measurements
of canopy height, percent canopy-cover, and percent shrub-cover in Vireo
breeding vegetation across all sites at each temperature-data logger location. We
took a mean of the 5 measurements for each vegetation metric at each temperaturedata
logger grid-point location. We also took the mean canopy height, percent
canopy-cover, and percent shrub-cover at nest sites. We used analysis of variance
(ANOVA, α = 0.05) to determine differences in mean vegetation characteristics at
the sites between the 2013 and 2014 Vireo breeding seasons. Lastly, we conducted
ANOVA on all vegetation metrics to assess differences between site vegetation at
temperature-data logger locations and nest-site vegetation duri ng each year.
Relationship of vegetation and ambient temperature. We used linear-regression
analyses to evaluate the relationships between temperature (mean, maximum, and
average maximum) and the associated vegetation measurements (canopy height,
and canopy and shrub cover) at temperature-data logger locations for each site during
the 2013 and 2014 breeding seasons.
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Temporal changes in nest characteristics. We employed linear regression for
each nest characteristic to determine if nest-vegetation characteristics changed
relative to time of season. To standardize analyses across the breeding season, we
analyzed all nest-site characteristics from nest-start date, defined as the day the first
egg was laid. If we located a nest with more than 1 egg or one with nestlings, we
estimated the nest-start date based on known Vireo nesting parameters. Vireos usually
lay 1 egg per day after the first egg is laid, and incubate eggs for ~15–17 d. After
hatching, nestlings remain in the nest for 10–12 d before fledging (Graber 1961).
We performed all analyses in the R statistical software program, version 2.15.3
(R Development Core Team, Vienna, Austria).
Results
Temperature
We deployed 18 temperature-data loggers from 15 April to 1 August 2013 that
recorded 44,723 hourly temperature readings. From 10 April to 31 July 2014, we
deployed 21 temperature-data loggers that recorded in 54,408 temperature readings.
Temperatures varied from -1.5 to 45 °C (mean = 24.2 °C, SD = 6.9) in 2013
and from -3 to 48 °C (mean = 24.3 °C, SD = 7.3) in 2014. The means for the 2
years were similar; thus, we combined temperature data across years for subsequent
analyses.
Site vegetation
We measured vegetation at 39 temperature-data logger locations during the
2013 (n = 18) and 2014 (n = 21) Vireo breeding seasons (Table 1). We found that
shrub-cover percentages and canopy height were significantly different at temperature-
data logger locations for the 2013 site than at both sites for 2014 (ANOVA:
F1, 37 = 4.16, P = 0.04; and F1, 37 = 4.29, P = 0.04; respectively). Shrub cover was
higher and canopy height was lower at the 2013 site than at both 2014 sites. However,
canopy cover was statistically similar between sites (ANOVA: F1, 37 = 0.57,
P = 0.45).
Ambient temperature and vegetation. We found no relationships between average
temperature, maximum temperature, or average maximum temperature at
temperature-data logger locations and average canopy cover, average canopy
height, or average shrub cover (linear regression: all P-values = 0.12–0.76; see
Holden 2016:figs. 2–4).
Table 1. Site vegetation metrics at temperature-data logger locations at Kerr Wildlife Management
Area, Kerr County, TX.
2013 (n = 18) site 2014 (n = 21) sites
Vegetation variable (units) Mean Min Max SD Mean Min Max SD
Canopy cover (%) 21.0 0.0 66.0 21.79 27.0 0.0 72.0 24.53
Shrub cover (%) 38.0 17.5 75.0 18.07 26.0 0.0 68.0 18.74
Canopy height (m) 3.0 0.0 10.0 2.73 5.5 0.0 11.0 3.61
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Nest-site vegetation
We measured vegetation at 181 Vireo nest sites during the 2013 (n = 55) and
2014 (n = 126) Vireo breeding seasons (Table 2). Nest height ranged from 0.2 m
to 4.0 m, with the mean of 1.0 m (Table 2). Nest height did not differ between
sites (ANOVA: F1, 179 = 1.52, P = 0.22). Mean shrub cover at Vireo nests was 47%
across years, with no large differences between years (ANOVA: F1, 179 = 3.291,
P = 0.07). Mean percent canopy-cover at nest sites was ~24% (SD = 17.7) across
years. Canopy cover at nest locations was significantly different between years
(ANOVA: F1, 179 = 4.12, P = 0.04) due to differences in vegetation at sites. In 2013,
mean percent canopy-cover was higher at nest sites than in 2014 (mean = 28.04%,
SD = 20.90 in 2013; mean = 22.22%, SD = 16.13 in 2014). Mean canopy height
above nests was 4.0 m (SD = 2.3; Table 2) and was similar between years (ANOVA:
F1, 179 = 0.42, P = 0.51).
We found that average nest canopy-cover and average site canopy-cover
(ANOVA: F1, 218 = 0.02, P = 0.89) and average nest canopy-height and average site
canopy-height (ANOVA: F1, 218 = 0.16, P = 0.69) did not differ. However, average
nest shrub-cover (~45%) was ~15% higher than average site shrub-cover (~30%;
ANOVA: F1, 218 = 21.41, P < 0.01; Holden 2016:fig. 5).
Temporal changes in nest characteristics
Over both study years, linear regression indicated there was no significant
change in mean nest height as the Vireo breeding season progressed (F1, 179 = 3.89,
P = 0.06), although the result was close to statistical significance. However, mean
canopy cover at nest sites decreased by ~15% (from ~35% to ~20%) over time
(F1, 179 = 5.16, P = 0.02). In contrast, there was an increase of approximately 15%
(~45% to 60%) in percent shrub cover over the breeding season (F1, 179 = 9.40, P less than
0.01). In addition, canopy height above nests decreased by an average of 1.5 m over
the course of the breeding season (F1,179 = 8.53, P < 0.01; Holden 2016:fig. 6).
Discussion
Although we cannot conclude that temperature was a limiting factor for Vireo
useable habitat space at Kerr Wildlife Management Area, we found that Vireos nested
in areas with different vegetation characteristics in relation to time of season. As
the useable-space hypothesis is stated, changes in an organisms’ needs should be
supported in respective habitat space (Guthery 1997). Vireos used areas of habitat
Table 2. Black-capped Vireo nest-vegetation metrics at Kerr Wildlife Management Area, Kerr
County, TX.
2013 (n = 55) site 2014 (n = 126) sites
Nest vegetation variable (units) Mean Min Max SD Mean Min Max SD
Nest height (m) 1.0 0.3 2.6 0.6 1.1 0.2 4.3 0.6
Canopy cover (%) 28.0 0.0 92.0 20.9 22.0 0.0 72.0 16.1
Shrub cover (%) 50.0 10.0 90.0 20.0 45.0 0.0 97.5 18.1
Canopy height (m) 4.0 0.0 11.0 1.9 4.5 0.0 12.0 2.4
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with higher amounts of shrub cover at nest sites than at the random temperaturedata
logger locations. We did not detect a difference in temperature within areas
with contrasting vegetation characteristics; however, there could still be some thermal
preferences by Vireos at nest sites that were not apparent because we could not
measure temperature at nest-site locations due to permit restrictions and because
we wanted to avoid disturbing nesting birds.
We found only a small difference in temperature between years, but the 2014
breeding season had a higher maximum temperature and lower average minimum
temperature. This difference in measured temperature between both years may be
due to differences in site vegetation in addition to annual variation. In previous
studies, temperature increases as small as 2−3 °C during bird breeding seasons
resulted in changes in reproductive success (Cox et al. 2013b). Furthermore, although
we did not study operative temperatures for the Vireo, evidence shows that
operative temperatures above 40 °C can be harmful for various bird species (Cox
et al. 2013b, Forrester et al. 1998). In our study, not only did maximum ambient
temperatures exceed 40 °C, but, the majority of data loggers placed across sites
experienced readings over 40 °C in both years.
Although Vireo habitat is managed with similar management treatments (i.e.,
cattle grazing, prescribed fire) across the KWMA, differences in time since last
treatment has resulted in varied vegetation characteristics across sites. Although
not statistically significant, the vegetation we measured at the 2013 site had higher
percentages of canopy cover but lower canopy heights than 2014 the sites. Site
vegetation and nest-site vegetation were very similar, but there was a difference
between nest-site shrub cover and site-vegetation shrub-cover percentages. Vireos
nested and used vegetation that was denser than surrounding areas within the site,
supporting previous claims that Vireos on the Edwards Plateau avoid more open
areas (Grzybowski et al. 1994). This observation could lead to further study about
foraging preferences, predator avoidance, and thermal needs of the species.
We could not identify a relationship between any vegetation characteristics and
the ambient temperature at our sites. Although we did not measure airflow, densely
packed vegetation in woodland interiors can cause a still and warm thermal environment
(Adams 2010). Thus, additional study is warranted to better understand the
relationship between temperature under higher percentages of canopy cover and the
amount of shrub or ground cover under the trees.
Differences in nest height were not statistically significant, but Vireos in our
study area placed nests lower as the breeding seasons progressed; nest height remained
within the known range (Conkling et al. 2012, Graber 1961, Grzybowski
1995). Furthermore, the tradeoff between avoiding predation and finding a thermal
refuge may also change during the breeding season, where predator avoidance
(including Brown-headed Cowbird predation on young) may prevail in the early
portion of the breeding season, but the need for thermal refugia prevails towards the
end of the breeding season, when temperatures begin to increase (Tieleman et al.
2008). Also, because Brown-headed Cowbird parasitism decreases as the breeding
season progresses (M.L. Morrison, unpubl. data), the birds might be able to later
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focus more on thermal factors rather than avoiding predators. Variation among individuals
adds additional factors to the study of predation risk versus thermal-refuge
preferences because body conditions and learned actions may cause individuals to
alter behaviors accordingly (Amat and Masero 2004). Colon et al. (2017) found
that during a drought, Vireos had lower pairing and territory success, delayed nestinitiation,
fewer re-nesting attempts, and lower nest-success relative to a more
moderate rainfall year.
We do not fully understand how expected climatic variation will change the
Vireo’s environment; however, we must consider the observed trends and adjust
management strategies accordingly. Shrublands with higher percent shrub-cover
may provide the necessary cover options and appropriate thermal refuge for Vireo
as seasonal temperatures increase. Our results suggest that Vireos may need shrubcover
options that are higher than previously recorded percentages (30−50%;
Grzybowski et al. 1994), particularly as temperatures increase over the course of
the Vireo breeding season. The availability of woodland vegetation may also provide
thermal cover in the absence of shrub cover, but an excess of woodland should
be avoided. This suggestion corresponds with previous studies that report Vireo
breeding habitat needs (Graber 1961, Grzybowski et al. 1994, Wilkins 2006).
Acknowledgments
We thank the Texas A&M University Natural Resources Institute for their financial and
logistical support and the Department of Wildlife and Fisheries Science staff and students
for their assistance. We especially thank the staff at Kerr Wildlife Management Area and the
Texas Parks and Wildlife Department for allowing site access, providing housing, and for
logistical support. We also thank Braun and Gresham, PLLC Texas EcoLab for their funding
as well as private landowners for allowing access to their properties. Two anonymous
referees provided valuable comments that helped to focus an ear lier draft.
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