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2018 SOUTHEASTERN NATURALIST 17(4)560–582
Tree Species Use and Seasonal Response to Food Availability
of Black-Capped Vireo
David T. Morgan1,3, M. Clay Green1,*, Michael L. Morrison2, and
Thomas R. Simpson1
Abstract - The ability of songbirds to survive and reproduce depends on many factors, one
of which is the ability to acquire enough food. We quantified foraging behavior, nestinghabitat
vegetation composition, and available arthropod prey of the Vireo atricapilla
(Black-capped Vireo) in Texas during 2010 and 2011. We used observational surveys of
foraging behavior and vegetation time-use to quantify the Black-capped Vireos’ foraging
behavior and vegetative use versus availability (i.e., mean proportion of use vs. vegetative
species availability). We collected descriptive data on the Black-capped Vireos’ foraging
use of available vegetative species and compared among vegetative species, year, and within-
season sampling periods. In 2010 and 2011, we identified and mapped 49 and 63 breeding
territories and repeatedly surveyed 30 and 58 territories for foraging activity, respectively.
Data analysis focused on the foraging use of the 3 most commonly used and available tree
species: Juniperus ashei (Ashe Juniper), Quercus sinuata (Shin Oak), and Q. fusiformis
(Live Oak). Ashe Juniper, Shin Oak, and Live Oak together made up 78.8% and 83.6% of
total proportion of substrate for foraging efforts in 2010 and 2011, respectively. Ashe Juniper
had the highest proportion (~28–50%) of foraging effort in 2010, 2011, and all but 1
sampling period for both years. We also repeatedly collected branch clippings from within
a random subset of surveyed Black-capped Vireo territories to identify potentially available
arthropod foods. We evaluated by order richness, total abundance, and dry biomass to make
comparisons among vegetative species, year, and within-season sampling periods. We found
significant differences in the biomass of arthropod orders Acari and Thysanoptera in 2010
and between orders Acari and Hymenoptera in 2011 among the 3 focal vegetative species.
Examination of additional descriptive data suggests seasonal changes in potentially available
arthropod foods. Our research underscores the importance of vegetation composition to
Black-capped Vireos that may help habitat managers select for potential vegetative species
distributions to optimize food resources throughout the breeding season for this species.
Introduction
Foraging opportunities and food availability for an insectivorous bird vary
in time and space and may shift on a seasonal or even daily basis depending on
weather, seasonal vegetation phenology, life-cycle, prey response or other variables
(Johnson and Sherry 2001, Orians 1980, Pyke et al. 1977). Studies have revealed
that some birds have the ability to seasonally track their foods, possibly knowing
which substrates are likely to yield key prey throughout the year (Hutto 1981, 1985;
1Wildlife Ecology Program, Department of Biology, Texas State University, San Marcos,
TX 78666. 2Department of Wildlife and Fisheries Sciences, Texas A&M University, College
Station, TX 77843. 3Current address - Power Engineers, Inc., 7600B North Capital of Texas
Highway #320, Austin, TX 78731. *Corresponding author - claygreen@txstate.edu.
Manuscript Editor: Michael Steinberg
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Johnson and Sherry 2001; Orians 1980). The ability to quickly identify these foraging
areas greatly reduces the amount of energy spent to obtain food, and potentially
increases the chances of survival and reproduction (Hutto 1990, Wolda 1990).
Plant phenology changes throughout the seasons, and arthropods may concentrate
on certain vegetative species at different periods of plant growth and development
(McGrath et al. 2008). Distinct preference for various tree and shrub species by
foraging birds is well documented (Gabbe et al. 2002, Holmes and Robinson 1981,
Wood et al. 2012). Therefore, the seasonal presence of certain vegetative species
likely influences the abundance and distribution of some bird sp ecies.
Vireo atricapilla Woodhouse (Black-capped Vireo; hereafter Vireo) is an, insectivorous
songbird whose breeding range extends from northern Mexico to central
Texas with disjunct populations in Oklahoma (Gonzalez-Rojas et al. 2014, Graber
1961, Ratzlaff 1987, USFWS 1991). The breeding range of this vireo once extended
north to Kansas, but human development, fire suppression, nest parasitism,
and over-grazing have caused the loss or degradation of habitat across the Vireo’s
range (USFWS 1991). Much of the current Vireo research is focused on identifying
and monitoring areas of breeding habitat (Benson and Benson 1990, Cimprich
and Kostecke 2006, Cooksey and Thompson 2005, Farquhar et al. 2003, Pinkston
et al. 2002) and threats to current populations (Eckrich et al. 1999, Guilfoyle
2002, Kostecke et al. 2005, Maresh 2005, Stake and Cimprich 2003). Other recent
research topics include breeding-habitat characteristics and nest-site selection (Bailey
2005, Dufault 2004, Greenman 1995, Grzybowski et al. 1994, Noa et al. 2007)
as well as the genetic variation of the species (Barr et al. 2008, Fazio 1994, Fazio
et al. 2004). Current literature is lacking critical information, however, on foraging
ecology and food availability of the Vireo (USFWS 1991, Wilkins et al. 2006).
There is little information about the diets of Vireos across their range. Graber
(1961) observed the Vireo to mainly glean insects from trees, primarily Quercus
(oaks). Graber (1961) also examined the stomach contents of 11 individuals and
found their diet to be similar to other vireo species. Studies conducted on the diets
of similar sized vireo species include Vireo griseus (Boddaert) (White-eyed
Vireo; Chapin 1925, Nolan and Wooldridge 1962), Vireo huttoni Cassin (Hutton’s
Vireo; Chapin 1925), and Vireo bellii Audubon (Bell’s Vireo; Chapin 1925,
Yard et al. 2004). These studies identified arthropods such as Araneae (spiders),
Diptera (flies/midges/mosquitoes), Hemiptera (true bugs), Hymenoptera (bees/
wasps/ants), Lepidoptera (butterflies/moths), and grasshoppers/crickets/katydids)
as potential vireo foods. Orthoptera (
To better understand the foraging ecology, food availability, and associated
management implications for the Vireo, our objectives were to: (1) conduct observational
foraging surveys of adult Vireos to quantify foraging methods and identify
temporal shifts in the use of foraging substrates throughout the breeding season,
(2) conduct observational behavioral surveys to identify and quantify temporal
changes of vegetation usage while foraging during the Vireo’s breeding season;
and (3) collect arthropod samples to document and track changes in the abundance
and composition of potentially available foods within habitat utilized by the species
throughout different periods of the breeding season.
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Study Areas
In 2010 and 2011, we studied Vireos in Travis, Burnet, and Williamson counties,
TX, in the Balcones Canyonlands National Wildlife Refuge (BCNWR). Located
within the Balcones Escarpment and Canyonlands eco-region in the southeastern
portion of the Edwards Plateau, the BCNWR contained 53 non-adjacent tracts of land
encompassing >8100 ha (USFWS 2001). We focused our research effort in Vireo
breeding territories on the Eckhardt (~413 ha), Rodgers (~1494 ha), Simons (~256
ha), Hiene (~24 ha), Russell (~39 ha), and Gainer (~236 ha) tracts. We selected these
tracts because they were known to host multiple breeding territories each year (Sexton
2002, 2005), and allowed us to sample across the refuge.
Vireo habitat on the BCNWR is typically classified as patchy shrubland comprising
mixed deciduous and evergreen vegetation of various heights, and dense
amounts of low-lying foliage (Morgan 2012, USFWS 2001). Shrub vegetation generally
varies from 1 m to 2 m in height with foliage extending nearly to ground level.
Mature trees >1.5 m in height, are often sparsely interspersed within or around the
edges of habitat areas. Vegetation usually consists of Quercus sinuata Buckl. (Shin
Oak) and/or Q. fusiformis Small (Plateau Live Oak) mixed with Juniperus ashei
J. Buccholz (Ashe Juniper), and Q. buckleyi Nixon & Dorr (Texas Oak) (Morgan
2012, USFWS 2001). Current Vireo habitat-management on BCNWR includes
selective habitat-disturbance practices such as prescribed burning and mechanical
habitat-manipulation to keep habitat in an early to mid-successional stage (USFWS
2001). Prescribed fire regimes vary based on the density of shrub vegetation, typically
a 5–10-y rotation. Mechanical habitat-manipulation uses manual clearing or
equipment such as a hydro axe to create an irregular habitat mosaic and prevent
the occurrence of a dense shrubland monoculture. Due to its invasive nature, Ashe
Juniper was historically removed or thinned from many of our study areas on the
BCNWR, so its density was sometimes relatively low within the Vireo management
areas. Additional habitat-management practices on BCNWR include parasitic
Molothrus ater (Boddaert) (Brown-headed Cowbird) population control, limitation
of human disturbance, and Odocoileus virginianus (Zimmermann) (White-tailed
Deer) herd management (USFWS 2001).
Average annual precipitation on the BCNWR is ~84 cm, with an average of
~39.5 cm from March to July (https://ncdc.noaa.gov), corresponding to the Vireo
breeding season. During both seasons of our study, precipitation totals from March
to July were below average, with ~33.8 cm occurring in 2010. The summer of 2011
experienced severe drought conditions with ~12 cm total precipitation from March
to July. The average annual temperature of this region is 19.4 °C, with an average
of 22.43 °C from March to July (https://ncdc.noaa.gov). During our study, mean
temperatures were above average in May and June in 2010, and all months of 2011.
Methods
Vegetation composition in Vireo territories
We searched for Vireos beginning at sunrise until ~13:00 CDT, walking transects
about 50–100 m apart while listening and watching for individuals (Bibby
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et al. 2000). Approximately half of the male vireos wore a unique color-band
combination to help identify separate individuals. These individuals had been
banded previously and during the years of this study as part of a larger effort to
study the Vireo on the BCNWR. When we located a Vireo, we spent a maximum
of 60 min observing the individual. We marked a GPS waypoint at the bird’s location
and continued to follow its movements, maintaining a minimum distance
of 20 m and recording additional waypoints each time the bird traveled ≥20 m
from its previous location. We designated a unique territory ID for territorial
waypoints and uploaded them into an ArcGIS 9.3.1 (ESRI 2009) point shapefile
and plotted accordingly. We recorded waypoints using the Universal Transverse
Mercator (UTM) grid system (NAD 1983, UTM Zone 14R). We revisited territories
every 3–10 d, with a minimum of 3 territory points marked during each
visit. We obtained territory points throughout the breeding season (March–July)
to adequately identify the occupied area. Territory-point locations for analysis
included all points where we observed Vireos conducting active behaviors when
they could be potentially foraging.
We conducted point-sampling vegetative surveys within territories beginning
in late June, after all territories were established and mapped. We used ArcGIS to
create minimum convex polygons (MCP) around the outer boundaries of identified
Vireo territories. We then used ArcGIS to create and overlay a 20 m x 20 m
sampling-point grid over the territory polygons. We surveyed each sampling point
within each territory MCP and determined if woody cover was present at the point.
If non-ground–based cover was present at the point (trees, shrubs, brush piles, or
snags) we recorded “yes”; if no cover was present (only rock, dirt, leaf litter, grass,
forbs) then we recorded “no”. If woody cover was present, we recorded the 3 most
prevalent woody species and minimum and maximum height of foliage cover to the
nearest 0.25 m.
Foraging ecology and vegetation use
During our study, we simultaneously used 2 types of observational surveys to
monitor foraging activities: (1) observational surveys to document sequential foraging
events, prey-capture foraging maneuvers, and use of vegetative substrates by
foraging Vireos throughout the breeding seasons; and (2) observational surveys to
document the amount of time spent in focal woody vegetative species while foraging.
We conducted both surveys between sunrise and 13:00 CDT from April to July
2010 and 2011, dividing the breeding seasons into 3 sampling periods. The early
sampling period (Period 1) occurred from mid-April to mid-May (12 April–10 May
2010, 12 April–11 May 2011) during the time Vireos are typically establishing territories,
searching for mates, and making early nesting attempts/incubation. The
middle sampling period (Period 2) occurred from mid-May to early June (11 May–1
June 2010, 12 May–12 June 2011) during the periods of peak nest incubation/nestling
activity. The late sampling period (Period 3) occurred from mid-June to early
July (2 June–29 June 2010, 13 June–11 July 2011), after nests had fledged young
and/or late nesting attempts had been made (Graber 1961). In 2011, during Periods
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2 and 3, Vireos had additional late nesting attempts when compared to 2010, which
extended sample period dates in 2011 a few days.
For the foraging and vegetative-use studies, we surveyed Vireo territories in the
same manner as described earlier in our nesting studies. We visited and attempted
to survey each territory every 3–10 d. Once we detected a Vireo, we maneuvered to
visually locate the bird without disturbing it, and, to minimize bias, waited a minimum
of 10 sec before recording data to ensure the bird had resumed normal activity
(Hejl and Verner 1990). We spent 30–60 min monitoring the individual visually,
with the aid of binoculars, and recording observational data. During this time, we
recorded data only during the time periods when the observer could accurately
determine the focal bird or mating pair’s behavior. Recording data sequentially
does not bias results given a sufficiently long recording period, as we used here
(e.g., Morrison 1984). We visually differentiated sexes using sexual dichromatism,
behavior, and audibly by vocalizations.
Observational surveys of foraging behavior. We classified a foraging event as
any instance when we observed a Vireo attempting to capture prey. We recorded
foraging maneuver, foraging surface, and species of vegetative foraging substrate.
We classified foraging maneuvers into 4 groups: (1) glean = to pick food item off
a substrate, no acrobatic movements involved; (2) hover-glean = to attack prey
while hovering in place; (3) sally = to fly from a perch to attack a food item and
then return to a perch; and (4) jump-glean = a leg-powered maneuver to attack from
a perch (Remses and Robinson 1990, Robinson and Holmes 1982). We classified
foraging substrates into 4 groups: (1) foliage = leaf, seed, flower, gall, or moss;
(2) branch = limb or offshoot from main stem; (3) trunk = main stem visibly distinct
from branches or roots; and (4) ground = bare soil, grass, leaf litter, or exposed root
(Morgan 2012, Robinson and Holmes 1982). We also recorded the estimated height
of each foraging event and minimum/maximum height of foliage cover in the foraging
substrate, all in 0.5-m increments. We also recorded any identifiable prey (size
and/or ID to order), sex of the foraging bird (male/female/unknown), territory ID,
and start time and end times.
Vegetation time-use surveys. Due to visibility constraints of the dense shrub
vegetation, we were not able to continuously observe all foraging events during a
survey; however, we were able to track individual Vireos’ movements using both
audible vocalizations and visual sightings. Sequential visual observations may
be biased towards foraging locations that are more conspicuous (Wagner 1981);
vegetation time-use surveys reduced this bias. As we followed individual Vireos,
we recorded the amount of time spent (min) in each woody vegetative species
for observed main behaviors. For statistical analysis, we separated active/mobile
behaviors into only 4 categories; (1) vocal = mobile individual vocally singing or
calling; (2) non-vocal = mobile individual without vocalizations; (3) territorial =
same sex, intraspecific interaction, and (4) courtship = male–female interaction less than 10
m, or display (adapted from Graber 1961; D.T. Morgan, pers. observ.). We observed
foraging events during all 4 of these behavior types. We recorded, but omitted
from statistical analysis, behaviors such as preening, perching, incubation, or other
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immobile behaviors when the Vireo could not be foraging. We also recorded estimated
minimum/maximum heights of foliage being used to the nearest 0.5 m. In
2010, we attempted to record vegetation time-use data for each territory visit for the
full 30–60 min. We were able to record behavioral data for approximately half
the survey time; we were unable to locate the Vireo for the other half of the visit.
Therefore, to simplify survey efforts and increase the number of surveys conducted
per territory in 2011, we attempted to record 24–30 min of observations over the
course of 3 visits (8–10 min each) for each territory.
Food availability. We sampled a random subset of known Vireo territories for
potentially available foods using the branch-clipping method (Cooper and Whitmore
1990). Available foods can be defined as, “the abundance of potential prey
items in microhabitats used by an insectivore when searching for food” (Wolda
1990:38). In 2010, we sampled 16 Vireo territories with 5 random points within
each (80 points total). In 2011, we sampled 20 Vireo territories with 3 random
points within each (60 points total). We sampled these same points once during
each of the 3 sampling periods to coincide with the foraging surveys. We obtained
samples over a 3–10-d period during the last half of each sampling period. We
sampled arthropods during the same daylight hours as foraging surveys. Given
that Vireos are primarily foliage-gleaning birds, we sampled potential foraging
substrates, specifically the outer foliage of available trees and shrubs (Graber 1961,
Grzybowski 1995, Houston 2008, Wolda 1990). We clipped branches from the most
dominant tree/shrub species within a 2-m radius of the random sampling point and
repeatedly sampled those same species each sampling period from that point. We
alternated available sampling heights varying from 0–1 m and 1–2 m. With as little
prior disturbance of the vegetation as possible, we selected, quickly enclosed in a
heavy-duty plastic sack, and removed a branch with hand shears. We immediately
placed a cotton ball dowsed with ethyl acetate or acetone into the sack and sealed
it to kill or stun arthropods inside and prevent predatory arthropods from feeding
on other captured prey. Within 1 h of collection, we placed sealed branches into a
cooler with ice packs to reduce decomposition and, within 4 h of collection, placed
them into a chest-style deep freezer at a temperature of 0 °C for a minimum of 72 h
to kill all remaining arthropods. After we removed branch samples from the freezer,
we examined each sample individually on a white surface to separate, collect,
count, and identify arthropods. We then sealed arthropods from each branch sample
in a new smaller bag and placed it back into the freezer for later identification, drying,
and weighing. We visually identified all arthropods to taxonomic order, which
allowed us to make acceptable comparisons in richness, biomass, and seasonal
changes (Cooper et al. 1990).
We further sorted orders Lepidoptera and Coleoptera to distinguish adult from
larvae and identified order Hemiptera to sub-order (Wolda 1990). After we removed
all arthropods, we labeled the remaining branch, placed it into a plant press to dry
and for subsequent weighing. Understanding potential available foods is critical to
understanding a species’ diet. Arthropod species richness, abundance, and biomass
of an area may originate from the diversity of available food resources and nutrient
or energy availability (Karr and Brawn 1990, Wolda 1990).
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When we compared stomach-content studies of similar vireo species (Beal
1907, Chapin 1925, Nolan and Woolridge 1962, Yard et al. 2004), we were unable
to classify any particular arthropod order as non-potential prey. However,
Sherry and McDade (1982) concluded that there was a direct rela tionship between
gape size and prey size in insectivorous, hover-gleaning birds. The average gape
width of the Vireo is ~5.6 mm (Rohwer and Spaw 1988). Therefore, we classified
potential prey as any hard-bodied (with a hardened sclerotized sheath) arthropods
≤5.6 mm in width and all soft-bodied (crushable) arthropods. We classified all
hard-bodied arthropods >5.6 mm wide as non-potential prey items and omitted
them from analysis. We identified, dried at a minimum of 50 °C for 72 h, and then
weighed to the nearest 0.001 g all potential prey items. We dried at 50 °C for 120
h then weighed all branch clippings to nearest 0.01 g.
Data analysis
We excluded from detailed statistical analysis data collected on female Vireos
due to the low sample size of female observations. Consistent with site-selection
criteria, we focused statistical analysis on the 3 most-prevalent vegetative species
within the territories surveyed (Ashe Juniper, Shin Oak, and Live Oak) for all foraging
and vegetation time-use data. Although we collected data on all vegetative
species observed, information on other species is only presented as a proportion
or percentage observed. Morgan (2012) provided a more detailed list of vegetative
species observed during our study.
Vegetation composition
To estimate percentage cover of available woody species, we determined the
sum of all woody species cover found at each sampling point in a territory and
divided the total for all species by each individual species in that territory. We
then averaged means across all territories to find percentage species cover for each
species. To estimate mean vegetation heights in each territory, we summed all
minimum and maximum heights separately for each species and divided by the total
number of points taken for that species. We averaged these minimum/maximum
means for all territories to estimate mean minimum and maximum vegetative species
heights. We used 2-sample t-tests to examine differences in mean proportion
of vegetative cover between reproductively successful and unsuccessful territories
for each sample year and each focal vegetative species (Sokal and Rohlf 2012). We
defined reproductively successful territory as having had at least 1 Vireo fledgling
visually identified outside of a nest in close proximity to the known nesting pair
during our survey.
Observational foraging surveys
To obtain cumulative percentages for 2010 and 2011, we calculated the total
number of each observed foraging maneuver and surface and then divided by
total number of each during each survey season. We did not compare between
sampling periods due to the low sample size of observed foraging maneuver.
We calculated the sum of the foraging-event heights within each vegetative species
and divided by the total number of foraging events within each territory. We
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averaged the vegetation height means across all territories to calculate mean and
standard error for foraging heights of observed foraging events. We calculated the
vegetative-species total proportion of foraging effort ([number of observed foraging
events in vegetative species] / [total observed foraging events]) for each year
and sampling period and calculated a foraging use vs. availability ratio ([percent
foraging effort in vegetative species] / [percent of vegetative species cover available];
Dodge et al. 1990, Thomas and Taylor 2006). We calculated foraging use
vs. availability ratios in each territory, for each focal vegetative species, and each
survey year. The ratios gave us a quantitative estimate of the amount of use of a
vegetative species compared to how much was available (e.g., a 2:1 ratio would
suggest a substrate was used proportionally twice as much as the percentage vegetative
cover available). We then used Pearson’s correlation coefficient (Sokal and
Rohlf 2012) to estimate correlation between total observation time and proportion
of foraging effort for these territories, and foraging use vs. availability ratio of
observed foraging events for each year, to ensure total amount of observation time
did not influence the proportion or ratios each year. We used one-way analysis of
variance (ANOVA) to compare differences in mean foraging use vs. availability
ratios between the 3 focal vegetative species in 2010 and 2011. We ran several
2-sample t-tests to compare mean proportion foraging effort per territory and use
vs. availability ratios between focal vegetative species, sample years, and reproductively
successful and unsuccessful breeding territories, and then calculated
95% confidence intervals for each analysis.
Vegetation time-use surveys
We calculated observed main behavior proportion ([total number of min of
observed behavior] / [total observation time]) for each year and sampling period.
For each focal vegetative species, we calculated proportion of vegetation time-use
([mean time in each vegetation species] / [mean total observation time]) for each
territory, during each year, and sample period. To compare proportion of timeuse
with the amount of vegetative species cover available in each territory, we
again calculated a time-use vs. availability ratio ([percent time-use in vegetative
species] / [percent of vegetative species cover available]) for each territory during
each year and sample period. We then used Pearson’s correlation coefficient
to estimate the correlation between territory total observation-time and territory
proportion of time-use and time-use vs. availability ratios for each year and
sample period. We used 1-way ANOVA to compare differences in mean time-use
vs. availability ratios between the 3 focal vegetative species in 2010 and 2011. We
ran several 2-sample t-tests to compare mean proportion time-use and time-use
vs. availability ratios between focal vegetative species, sample years, and reproductively
successful and unsuccessful breeding territories and then calculated
95% confidence intervals for each analysis.
Food availability
We calculated the mean arthropod abundance (total number of individuals collected),
mean biomass (milligrams of dried arthropods collected), and mean order
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richness (number of orders observed) for each branch clipping obtained. We also
calculated mean arthropod abundance, mean arthropod biomass, and mean order
richness for each focal vegetative species, each sample year, and sample period.
We used Pearson’s correlation coefficient to estimate potential correlation between
branch-clipping woody biomass and order richness, arthropod abundance, and
arthropod biomass to ensure mass of branch collected did not influence diversity,
abundance, or biomass of arthropods collected. For each of the 10 most-common
arthropod orders, we used a 1-way ANOVA to examine differences in mean arthropod
biomass between the 3 focal vegetative species. We used several 1-way
ANOVAs to compare arthropod order richness, abundance, and biomass between
sample periods and sample year for each focal vegetative species; all means were
calculated with standard error (SE). We used 2-sample t-tests to compare mean
arthropod abundance and biomass between territory reproductive success, for
each year in each focal vegetative species. We conducted all statistical analysis in
program R 2.10.1 (R Development Core Team, Vienna, Austria) and tested the assumptions
of normality for parametric tests (Sokal and Rohlf 2012).
Results
Vegetation composition
We identified and mapped 49 and 63 breeding territories and repeatedly surveyed
30 and 58 territories for foraging activity during 2010 and 2011, respectively.
Mean (± SE) territory size was 2.5 ha ± 2.71 and 3.1 ha ± 2.12 in 2010 and 2011,
respectively. Territory mean (± SE) woody cover was 86.0% ± 9.37 in 2010 and
78.5% ± 15.61 in 2011. Dominant canopy-cover tree species included Shin Oak
(2010: 24.8% ± 13.64, 2011: 24.0% ± 17.27), Ashe Juniper (2010: 14.8% ± 11.71,
2011: 21.8% ± 17.27), Live Oak (2010: 3.3% ± 5.71, 2011: 6.7% ± 10.09), Rhus
lanceolata (A. Gray) Britton (Flame-leaf Sumac) (2010: 3.6% ± 4.43, 2011: 4.7% ±
8.17), and Texas Oak (2010: 3.3% ± 5.03, 2011:3.0% ± 3.70). Morgan (2012) provided
a full list of vegetative species and amount of woody cover within territories
during our study.
Observational foraging surveys
Gleaning was the primary foraging maneuver observed during foraging events
in both 2010 (75.5%, n = 206) and 2011 (64.7%, n = 244). Followed by hover-glean
(2010: 19.1%, n = 52; 2011: 21.8%, n = 82), jump-glean (2010: 1.5%, n = 4; 2011:
8.2%, n = 82), and sally (2010: 4.0%, n = 11; 2011: 5.3%, n = 20). Vegetative foliage
was the primary foraging substrate for males (2010: 80.1%, n = 169; 2011:
77.3%, n = 259), followed by branch (2010: 18.0%, n = 38; 2011: 21.2%, n = 71),
ground (2010: 1.9%, n = 4; 2011: 0.6%, n = 2), and trunk (2010: 0.0%, n = 0; 2011:
0.9%, n = 3).
In 2010, mean (± SE) height of observed foraging event for all vegetation species
was 2.6 m ± 1.38 (n = 283) with a mean minimum foliage height of 0.4 m ±
0.63 and a mean (± SE) maximum foliage height of 3.8 m ± 1.50. In 2011, mean
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(± SE) height of observed foraging event for all vegetation species was 2.7 m ± 1.55
(n = 378) with a minimum foliage height of 0.6 m ± 0.78 and a maximum foliage
height of 4.2 m ± 1.63.
Ashe Juniper, Shin Oak, and Live Oak together made up 78.8% of total proportion
of foraging events in 2010 and 83.6% in 2011 (Fig. 1). Mean foraging use
vs. availability ratio was 1.80 in 2010 and 2.71 in 2011 for Ashe Juniper, 1.21 in
2010 and 0.79 in 2011 for Shin Oak, and 4.64 in 2010 and 2.26 in 2011 for Live
Oak. There were significant differences in mean foraging use vs. availability ratios
between the 3 focal vegetative species in 2010 (F2,70 = 3.527, P = 0.035) and 2011
(F2,141 = 6.643, P = 0.002).
Ashe Juniper. Ashe Juniper had the highest proportion of foraging effort in both
2010, 2011, and all but 1 sampling period, for both years (Fig. 1). We found no significant
correlation between total observation time and proportion of foraging effort
(2010: r = 0.093, 2011: r = -0.271) or foraging use vs. availability ratio (2010: r =
-0.19, 2011: r = -1.114) during sample years or sampling periods. Ashe Juniper’s
foraging use vs. availability ratio in 2010 was 1.80 ± 0.61 and 2.71 ± 0.89 in 2011
(t82 = -1.39, P = 0.167). Mean foraging use vs. availability ratios were not significantly
different between reproductively successful and unsuccessful territories in
2010 (t127 = 0.22, P = 0.831), but there was significant difference between successful
Figure 1. Observed proportion of foraging effort (number of observed foraging events in
vegetative species/total observed foraging events) within focal vegetative species used by
male Vireo atricapilla (Black-capped Vireos) at Balcones Canyonlands National Wildlife
Refuge, TX, 2010–2011. Sampling periods: Period 1 occurred from mid-April to mid-May;
Period 2 occurred from mid-May to early June; Period 3 occurred from mid-June to the early
July; and n = number of observed foraging events observed.
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and unsuccessful territories in 2011 (t53 = 2.20, P = 0.032). Mean foraging use vs.
availability ratio was 1.5 for territories that fledged young (n = 22) and 3.5 for unsuccessful
territories (n = 36). Alternatively, mean vegetative cover of Ashe Juniper
did not differ between successful and unsuccessful territories in 2010 (t27 = -0.56,
P = 0.578) or 2011 (t53 = -1.09, P = 0.281).
Shin Oak. We found no significant correlation between total observation time and
proportion of foraging effort (2010: r = -0.207, 2011: r = 0.328) or foraging use vs.
availability ratio (2010: r = -0.485, 2011: r = 0.237) during sample years or sampling
periods. Shin Oak’s availability ratio was 1.2 ± 0.71 in 2010 and 0.8 ± 0.34 in 2011
(t77 = 1.252, P = 0.214). Mean foraging use vs. availability ratios were not significantly
different between territories that had reproductively successful Vireos and those
that did not in 2010 (t26 = -0.302, P = 0.765) or 2011 (t49 = -1.217, P = 0.230). Mean
Shin Oak vegetative cover did not differ between successful and unsuccessful territories
in 2010 (t26 = -1.196, P = 0.242) or 2011 (t49 = 0.796, P = 0.191).
Live Oak. We found no significant correlation between total observation time
and proportion of foraging effort (2010: r = -0.101, 2011: r = -0.02) or foraging
use vs. availability ratio (2010: r = -0.262, 2011: r = 0.222) during sample years or
sampling periods on Live Oak. Live Oak’s availability ratio in 2010 was 4.6 ± 4.57
and 2.3 ± 1.14 in 2011 (t52 = 1.468, P = 0.148). Mean foraging use vs. availability
ratios were not significantly different between territories that had reproductively
successful Vireos and those that did not in 2010 (t14 = 0.474, P = 0.643) or 2011
(t36 = 0.201, P = 0.842). There were no significant differences between mean Live
Oak vegetative cover with respect to reproductive success in 2010 (t14 = 1.072,
P = 0.302), but mean vegetative cover did differ in 2011 (t36 = -2.379, P = 0.023).
In 2011, mean (± SE) Live Oak cover was 4.5% ± 2.00 for territorrites with no
reproductively successful Vireos (n = 36) and 10.4% ± 6.62 for territories with
reproductively successful Vireos (n = 22).
Vegetation time-use surveys
In 2010 we repeatedly sampled 30 territories and recorded 2035 min of male
behavioral observations. In 2011, we repeatedly sampled 58 territories and recorded
3232 minutes of male behavioral observations. In 2010 and 2011, the
mean (± SE) number of minutes recorded per territory was 80.5 ± 36.43 and
58.4 ± 28.21, respectively. Ashe Juniper, Shin Oak, Live Oak, Texas Oak, dead
vegetation, and Celtis occidentalis L. (Hackberry) combined formed the highest
proportion of time ([time in species] / [total time]) spent within vegetative species
in 2010 (92.68%) and 2011 (88.38%) (Fig. 2). Ashe Juniper, Shin Oak, and Live
Oak, alone, made up 68.67% of total proportion of time-use in 2010 and 76.69%
in 2011 (Fig. 2). Mean time-use vs. availability ratio was 2.13 in 2010 and 1.91
in 2011 for Ashe Juniper, 1.15 in 2010 and 1.35 in 2011 for Shin Oak, and 4.18 in
2010 and 4.23 in 2011 for Live Oak. Means differed in time-use vs. availability
ratios between the 3 focal vegetative species in 2010 (F2,74 = 8.417, P < 0.001) and
2011 (F2,156 = 6.463, P = 0.002).
Ashe Juniper. We found no significant correlation between total observation
time and proportion of time-use (2010: r = -0.004, 2011: r = -0.231) or time-use
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vs. availability ratios (2010: r = -0.434, 2011: r = 0.114) during sample year or
sample period. Mean time-use vs. availability ratios did not differ between years
(t86 = 0.654, P = 0.515), sample periods in 2010 (F2,65 = 0.054, P = 0.947), or sample
periods in 2011 (F2,133 = 2.252, P = 0.109). There was no difference in mean Ashe
Juniper time-use vs. availability ratios in relation to territory reproductive success
in 2010 (t28 = 0.015, P = 0.988), but there were differences in mean use vs. availability
ratios in 2011 (t56 = 3.244, P =0.002). Successful territories in 2011 (n = 22)
had a mean time-use vs. availability ratio of 1.304 (SE = 0.32), while unsuccessful
territories (n = 36) had a mean ratio of 2.273 (SE = 0.43) (Morgan 2012).
Figure 2. Vegetation time-use proportion ([number of observed vegetation min] / [total
min]) by male Vireo atricapilla (Black-capped Vireos) and total minutes of observation at
Balcones Canyonlands National Wildlife Refuge, TX, 2010–2011.
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Shin Oak. We found no significant correlation between total observation time
and proportion of time-use (2010: r = 0.029, 2011: r = 0.064) in 2010 or 2011. We
found no significant correlation between time-use vs. availability ratios (2010: r =
-0.041, 2011: r = -0.113) during 2010 or 2011 or during any sample period. There
were no differences in mean time-use vs. availability ratios between sample years
(t81= -0.765, P = 0.446). There were differences between sample periods in 2010
(F2,64 = 5.985, P = 0.004), but no differences between sample periods in 2011 (F2,126
= 0.215, P = 0.807). There were no significant differences in mean time-use vs.
availability ratios in relation to territory reproductive success in 2010 (t27 = -0.813,
P = 0.424) or 2011 (t52 = -1.650, P = 0.105).
Live Oak. We found no significant correlation between total observation time
and proportion of time-use (2010: r = -0.678, 2011: r = -0.212) or time-use vs.
availability ratios (2010: r = -0.196, 2011: r = 0.022) during 2010 or 2011 or during
any sample period. Mean time-use vs. availability ratios did not differ between
sample years (t63 = -0.030, P = 0.977), sample periods in 2010 (F2,46 = 1.652, P =
0.203), or 2011 (F2, = 0.054, P = 0.819). There were no significant differences in
time-use vs. availability ratios in relation to territory reproductive success in 2010
(t16 = 0.266, P = 0.793) or 2011 (t45 = 0.717, P = 0.477).
Food availability
We analyzed the 10 most-prevalent arthropod orders from our branch-clipping
samples. These included Acari, Araneae, Coleoptera, Diptera, Hemiptera, Hymenoptera,
Lepidoptera, Neuroptera, Orthoptera, and Thysanoptera. The only significant
differences in biomass between the 3 focal vegetative species was for orders Acari
and Thysanoptera in 2010 and Acari and Hymenoptera for 2011 (Table 1).
Ashe Juniper. We found no significant correlations between sample-branch
weight and arthropod abundance (2010: r = 0.101, 2011: r = 0.094) or biomass
(2010: r = 0.036, 2011: r = -0.025). We found significant differences in mean order
richness between sample periods in 2 2011 (Table 2), but no significant differences
in relation to territory reproductive success in 2010 (t30 = 0.730, P = 0.471) or 2011
(t24 = -1.682, P = 0.106). We found no significant differences in mean arthropod
abundance between sample periods in 2010 or 2011 (Table 2). Although in 2010
there were no differences in mean arthropod abundance in relation to territory
success in 2010 (t30 = 0.056, P = 0.956), there were significant differences in 2011
(t24 = -2.136, P = 0.043). Successful territories averaged 3.31 arthropods per branch
clipping, while unsuccessful territories only averaged 1.81 arthropods per clipping.
We found no differences in mean arthropod biomass between sampling periods in
2010 or 2011 (Table 2). There were no differences in mean arthropod biomass in
relation to territory reproductive success in 2010 (t30 = 1.470, P = 0.152) or 2011
(t24= -0.225 P = 0.824).
Shin oak. We found no significant correlations between sample-branch weight
and arthropod abundance (2010: r = 0.039, 2011: r = -0.099) or biomass (2010:
r = 0.0457, 2011: r = -0.007). We found no significant differences in mean order
richness between sample periods in 2010 or 2011. We found no differences in
mean richness in relation to territory reproductive success in 2010 (t54 = 1.167,
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P = 0.248), or 2011 (t31 = -1.556, P = 0.130), though successful territories averaged
1.21 orders per branch clipping, while unsuccessful territories averaged only 0.85
orders overall for both years. We found no significant differences in mean arthropod
abundance between sample periods in 2010 or 2011 (Table 2). There were no differences
in mean abundance in relation to territory success in 2010 (t54 = 1.420, P =
0.162) or 2011 (t31 = 0.505, P = 0.617). We found no differences in mean arthropod
biomass between sampling periods in 2010 or 2011. There were also no differences
in mean arthropod biomass in relation to territory reproductive success in 2010
(t54 = 0.754, P = 0.454) or 2011 (t31 = -0.308, P = 0.760).
Live Oak. We found no significant correlations between sample-branch weight
and arthropod abundance 2010: r = 0.216, 2011: r = -0.177) or biomass (2010: r =
0.099, 2011: r = -0.195). We found no significant differences in mean order richness
between sample periods in 2010 and 2011 or significant differences in mean order
richness in relation to territory reproductive success in 2010 (t16 = 0.601, P = 0.556)
or 2011 (t14= 1.475, P = 0.162). We found no significance differences in mean
Table 1. Comparison of mean arthropod biomass among focal vegetative species within Vireo atricapilla
(Black-capped Vireo) breeding territories at Balcones Canyonlands National Wildlife Refuge,
TX, 2010–2011. Data represents mean dry arthropod biomass (mg) per branch clipping, n = number
of samples. Asterisk (*) indicates significance (P < 0.05).
Mean biomass (mg)
Order Ashe Juniper Shin Oak Live Oak) F df P
2010
n 96 168 54
Acari 0.625 0.179 0.000 3.195 2 0.042*
Araneae 18.333 14.464 7.037 1.351 2 0.260
Coleoptera 5.729 6.548 16.481 1.530 2 0.218
Diptera 2.188 1.548 1.667 0.396 2 0.673
Hemiptera 14.792 7.262 8.889 0.841 2 0.432
Hymenoptera 1.979 3.274 4.259 0.570 2 0.556
Lepidoptera 20.625 6.310 49.440 2.622 2 0.074
Neuroptera 0.313 0.119 0.000 0.891 2 0.411
Orthoptera 32.917 13.929 0.000 0.929 2 0.396
Thysanoptera 0.104 0.595 0.000 3.412 2 0.034*
All Orders 10.469 5.530 13.815 2.041 2 0.131
2011
n 78 99 48
Acari 3.846 0.202 0.208 17.228 2 < 0.001*
Araneae 10.380 12.929 16.667 0.234 2 0.791
Coleoptera 1.795 0.303 3.958 1.719 2 0.182
Diptera 5.897 0.101 0.000 1.104 2 0.333
Hemiptera 6.282 7.172 11.458 0.495 2 0.610
Hymenoptera 1.154 8.586 4.167 4.507 2 0.012*
Lepidoptera 1.282 0.606 2.500 2.746 2 0.066
Neuroptera 0.000 0.000 0.000 NA - NA
Orthoptera 1.026 0.202 1.458 1.470 2 0.232
Thysanoptera 0.128 0.303 0.000 0.929 2 0.396
All Orders 3.333 3.242 4.167 0.267 2 0.766
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Table 2. Comparison of mean arthropod order richness (number of collected orders), total abundance (number of collected individuals), and total biomass
(mg of dry arthropods collected) among focal vegetative species branch-clippings taken within Vireo atricapilla (Black-capped Vireo) breeding territories
at Balcones Canyonlands National Wildlife Refuge, TX, 2010–2011. Asterisk (*) indicates significance (P < 0.05).
2010 2011 Season Totals
Species Period 1 Period 2 Period 3 F P Period 1 Period 2 Period 3 F P 2010 2011 F P
Ashe Juniper (n = 32) (n = 32) (n = 32) (n = 26) (n = 26) (n = 26) (n = 96) (n = 78)
Richness 2.03 1.44 1.28 2.72 0.071 1.62 0.77 1.04 5.72 0.005* 1.58 1.14 6.02 0.015*
Abundance 3.28 2.56 1.91 1.91 0.154 2.85 2.00 2.65 0.60 0.552 2.55 2.49 0.02 0.882
Biomass 5.40 12.30 13.80 0.78 0.463 4.20 4.00 1.90 0.87 0.423 10.47 3.33 4.64 0.036*
Shin Oak (n = 56) (n = 56) (n = 56) (n = 33) (n = 33) (n = 33) (n = 168) (n = 99)
Richness 1.36 1.55 1.25 0.94 0.393 1.03 0.94 0.85 0.38 0.686 1.39 0.94 10.77 0.001*
Abundance 1.93 2.98 1.82 2.58 0.079 1.79 2.15 1.09 1.12 0.332 2.22 1.67 2.19 0.140
Biomass 3.50 8.10 4.90 1.66 0.193 3.10 4.50 2.10 0.82 0.444 5.53 3.24 2.31 0.130
Live Oak (n = 18) (n = 18) (n = 18) (n = 16) (n = 16) (n = 16) (n = 54) (n = 48)
Richness 1.50 1.17 1.28 0.41 0.666 1.00 1.31 0.81 0.96 0.391 1.32 1.04 1.62 0.206
Abundance 2.00 1.61 2.67 1.20 0.310 3.50 2.44 1.38 0.62 0.544 2.09 2.44 0.19 0.665
Biomass 4.20 15.83 11.50 0.64 0.531 4.70 5.40 2.40 0.70 0.502 10.50 4.17 1.87 0.175
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arthropod abundance between sample periods in 2010 and 2011 or in relation to reproductive
success in 2010 (t16 = 0.801, P = 0.435) or 2011 (t14 = 0.328, P = 0.748).
We found no differences in mean arthropod biomass between sampling periods in
2010 or 2011. There were also no differences in arthropod biomass in relation to
territory reproductive success in 2010 (t16 = 0.123, P = 0.904) or 2011 (t14 = 1.479,
P = 0.161).
Discussion
Our data provides similar results to those reported in documented resources
available on the foraging ecology of the Vireo. Gleaning is thought to be an energetically
inexpensive means of obtaining prey (Remsen and Robinson 1990) and
allows the birds to find smaller hidden foods that may not be found with other
searching methods (Robinson and Holmes 1982). Grzybowski (1995) stated that
Vireos did not solely forage in shrub habitat, but rather foraged in a variety of vegetation
heights, which supports Robinson and Holmes’ (1982) findings that birds
foraging at different vegetation layers were exposed to more foraging opportunities
and available foods. We also found that male vireos foraged at a variety of vegetation
heights. Our research revealed some interesting findings on vegetation usage,
notably the higher proportion of use of Ashe Juniper and Live Oak during both
foraging and vegetation time-use observations.
Shin Oak, Ashe Juniper, and Live Oak represent the most prevalent tree species
within Vireo territories in our study. While the percent of Shin Oak vegetative cover
was similar to what has been reported in other Vireo studies, the percent cover of
Ashe Juniper was greater than what other studies have observed within the Edwards
Plateau and Lampasas Cut Plain eco-regions (Grzybowski et al. 1994, Tazik et al.
1993, USFWS 1991). Our estimates are similar to those for the structural characteristics
of breeding habitat described by the USFWS (1991), including Vireo habitat
deciduous cover at 0–3 m in height, with 30–50% cover. Grzybowski et al. (1994)
observed Ashe Juniper cover levels at ~8%, whereas, we observed 14–22% mean
Ashe Juniper cover within our study areas. Although Live Oak and Shin Oak are
considered primary oak species for Vireos (Grzybowski et al. 1994), we observed
low mean vegetative cover of these oak species within our Vireo territories.
While Shin Oak, Live Oak, and Ashe Juniper were the prevalent tree species
found within territories, each was used differently by foraging Vireos. On
average, Shin Oak provided the most abundant available shrub cover within the
territories, proportionally, we observed Shin Oak to be the second most-popular
foraging-event substrate in 2010 and 3rd in 2011. Of the 3 focal species, Shin Oak
had the lowest foraging use vs. availability ratio in both 2010 and 2011. Shin Oakdominated
habitats were scattered with Ashe Juniper, Live Oaks, and/or Texas Oaks
of greater maturity. Rhamnus caroliniana Walt. (Carolina Buckthorn), Cercis spp.
(redbuds), Rhus spp. (sumacs), and Diospyros texana Scheele (Texas Persimmon)
were mixed in with the Shin Oak, although we seldom observed foraging activities
within these species. However, we did find Vireo nests in their branches (Morgan
2012). We also observed Vireo use on another Shin Oak vegetative type: mature
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Shin Oak/Ashe Juniper woodland, usually near the edge of shrub habitats or Vireo
management areas. Shin Oak/Ashe Juniper woodlands, more commonly associated
with Setophaga chrysoparia (Sclater & Salvin) (Golden-cheeked Warbler; USFWS
1992), have been reported to be occupied by Vireos as well (Graber 1961).
Mean vegetative cover of Live Oak was low, but we observed a higher proportion
of foraging effort, foraging use vs. availability ratios, and vegetative time-use
vs. availability ratios within the species. Live Oak had one of the highest foraging
and time-use vs. availability ratios when compared to Ashe Juniper and Shin Oak.
Although, Live Oak mean time-use vs. availability ratios did not significantly differ
between sample periods. Our study indicated that when Live Oak was found in Vireo
habitat, it was used in a much greater proportion than its availability. While this
use pattern may be due to greater food availability associated with Live Oaks than
other tree species, the foliage structure of Live Oaks may also explain the high use
vs. availability. Gleaning species, like the Vireo, have been shown to have strong
preferences for particular tree species that have foliage evenly distributed along the
twigs and branches (Holmes and Robinson 1981). Houston (2008) also observed a
great amount of foraging activity in Live Oak. Similarly, food accessibility, which
is affected by plant structure (e.g., leaf distribution and petiole length) can influence
foraging success and, therefore, foraging preference of foliage-gleaning birds for
specific tree species (Robinson and Holmes 1982, Wood et al. 2012).
Ashe Juniper was the only vegetative species found in all our Vireo territories
during both years and it was not uncommon to find Vireo nests in Ashe Juniper
branches. The majority of the Ashe Juniper trees used during our study were within
older oak/juniper woodland on the edge of recently managed areas, which may
have contributed to the higher estimates for Ashe Juniper cover. We also typically
observed Vireos using tall, mature Ashe Juniper trees existing within shorter, dense
Shin Oaks or Ashe Junipers established underneath Live Oak canopies. Some of
these observations were revealed in the observed mean maximum vegetativespecies
heights recorded during observed foraging events. Of all observed male
foraging-events, mean Ashe Juniper maximum height was ~1.5 m taller than mean
Shin Oak maximum height and ~1.5 m less than mean Live Oak maximum height
during both years. Our data showed the frequent use of Ashe Juniper is likely related
to the abundance of available foods, protective cover, and singing perches.
Grzybowski et al. (1994) observed that Vireos are able to tolerate areas with a large
amount of Ashe Juniper canopy cover, but are likely to be found in areas of low
Ashe Juniper cover. However, we did not find any differences between amount of
Ashe Juniper cover and reproductive success (Morgan 2012). Quinn (2000) and
Marshall et al. (2013) both found an increasing number of total arthropods in Ashe
Juniper during late May and June, a time when other tree species are showing a
declining number of caterpillars.
When we compared arthropod biomass in 2010 and 2011 among the 3 focal
vegetative species, there seemed to be an obvious effect from the drought. However,
there was little difference in mean arthropod abundance between 2010 and
2011, mainly due to abundant low-biomass arthropods like mites, aphids, ants,
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and immature spiders that were more common in 2011. In each vegetative species
and every sample period, there was lower arthropod biomass during 2011 than
2010. Also, in 2011, we observed lower total order richness in each vegetative species,
with significant differences in Ashe Juniper and Shin Oak. Kendeigh (1970)
proposes that birds may pursue prey of a certain size that yields a food with a
value at least equal to the energy used for locating and consuming it. This lack of
arthropod biomass but increase in abundance during the drought may coincide with
an increase in foraging-energy expenditure. Some of these arthropod species may
impact the Vireos on different trophic levels, and abundance of these minute insects
during the drought may not have necessarily equated to more prey. Chapin (1925)
found only 1 instance of mites in his examination of over 1900 stomach samples
from 8 species of vireos. However, Lehman (1982) observed that mites may be an
important prey species for other predatory arthropods.
Graber (1961) observed examples of Vireos utilizing surface waters, if available,
for consumption. However, that author noted that the presence of surface waters is
not a habitat requirement and concluded that a majority of the bird’s water intake
is obtained through their food sources, specifically larval-stage insects. No perennial
surface waters were located within any of the observed territories in our study
and we did not observe Vireos leaving their territories to visit any water sources.
Occasionally, we observed individuals utilizing water from dew or rain that has collected
on leaves or as puddles. Although we did not evaluate water-mass content of
collected arthropods, our study suggests that a majority of the birds’water intake is
obtained metabolically through their food sources.
We identified most of the arthropod orders we collected during our study as
known food sources for similar vireo species (Chapin 1925, Nolan and Wooldridge
1962, Yard et al. 2004). We directly observed Vireos capturing a variety of arthropods
throughout the study, including grasshoppers, katydids, caterpillars, and
spiders to eat and/or feed their young.
Seasonal changes in arthropod life cycles may affect Vireo foraging habits. Araneae
may be a significant food source, especially later in the breeding season when
other food sources are declining due to dryer and warmer summer weather patterns.
Dipterans can be very abundant under the right weather conditions. Many species of
dipterans are aquatic during the larval stage (Eaton and Kaufman 2007); therefore,
their low numbers in 2011 may be explained by the severe drought.
Order Lepidoptera—butterflies and moths—are known as one of the most important
food sources for many vireo species (Chapin 1925, Nolan and Wooldridge
1962, Yard et al. 2004), including the Black-capped Vireo (Graber 1961). Lepidopterans
and their larvae (caterpillars) are a critical food source for nesting Vireos
to feed to their young (Graber 1961). These larvae typically emerge in abundance
during the peak nesting periods. Graber (1961) observed Lepidoptera in 10 of 11
Vireo stomachs, comprising from 30 to 85% of their total contents. Most of the
Lepidoptera we collected were larvae, 95.5% in 2010 and 92.3% in 2011 (Morgan
2012). Our study showed a noticeable decrease of biomass of Lepidoptera in 2011
compared to 2010, during all sample periods. In 2010, we found Lepidopterans in
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all 3 focal vegetative species during all sample periods, and each species observed
had relatively high mean biomass of Lepidoptera larvae at certain times of the
season. In 2011, caterpillars were non-existent during several sampling periods.
Live Oak samples contained the highest mean biomass of caterpillars both years.
We identified the most prevalent lepidopteron larvae found on Ashe Juniper as
Cudonigera houstonana Grote (Juniper Budworm). The emergence of these moth
larvae in April and May has been observed to be a significant food source for the
Golden-cheeked Warbler in Central Texas (Quinn 2000).
Current recommendations for management of Vireo habitat suggest vegetation
composition is less important than plant structure (e.g., presence of deciduous
shrubs with foliage extending to the ground; Campbell 2003, USFWS 1991).
Our research shows that the composition and spatial juxtaposition of woody
vegetative species also plays an important role in providing optimum foraging
opportunities. Also, based on our results, we recommend that Ashe Juniper be
managed to maintain 10–25% cover; an increase from previous recommendations
of less than 10% (Grzybowski et al. 1994). We conclude that Ashe Juniper should
not be allowed to form thick monocultures. Structurally, Ashe Juniper may be
most beneficial for Vireos in small, scattered clumps, mixed within or abutting
deciduous shrub vegetation, or as the understory of mature canopy trees, and if
possible, with branches extending near to the ground. Ashe Juniper should provide
Vireos with foraging opportunities for Lepidopterans, Coleopterans, and
Dipterans during the first half of the breeding season; Araneae, Orthopterans,
and Hemipterans during the second half of the season; and variable amounts of
other arthropods throughout the breeding season.
Based on our observations, we also recommend managing Shin Oak cover
to about 25–50% within habitat for Vireos, with heights varying from 1.5 m to
branches extending to the ground. Shin Oak should provide Vireos with foraging
opportunities for Lepidopteran larvae early in the breeding season, Araneae during
the middle to late breeding seasons, and a variation of other arthropod orders
including Hemipterans, Hymenopterans, and Coleopterans throughout the breeding
season. We suggest managing Live Oak cover at 5–25%, mainly consisting of mature
large canopy trees, if possible, with a low deciduous or evergreen understory
and branches extending low to the ground. We also recommend that Live Oak
be dispersed throughout the habitat and not in large continuous blocks to ensure
Live Oak is available to multiple territories. Live Oak should provide Vireos with
early season foraging opportunities for Dipterans, mid-season opportunities for
Hymenopterans, and mid-late foraging for Lepidopterans and Coleopterans. Live
Oak should also provide variable numbers of Araneae and Hemipterans throughout
the breeding season. Morgan (2012) provided a full comparison of sample-period
mean arthropod-order richness, total abundance, and total biomass for each of the
3 focal woody species. Along with Ashe Juniper, Shin Oak, and Live Oak, we also
recommend the growth of a diversity of deciduous woody species, including but
not limited to, redbuds, sumacs, Carolina Buckthorn, Texas Persimmon, Hackberry,
and Texas Oak. We observed foraging and nesting attempts in each of these species.
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Acknowledgments
We thank the Texas A&M Natural Resources Institute and the members of the Texas
A&M RAMSES research group for their helpfulness and insight as well as their financial
and logistic support. We also thank the staff of the Balcones Canyonlands National Wildlife
Refuge for allowing us access, resources, and housing during both field seasons. We
are grateful to the Texas Department of Transportation and the Houston Safari Club whose
financial support made this all possible. We thank several anonymous reviewers for constructive
reviews on earlier versions of this manuscript. Finally, we extend a special thanks
to Erin Cord for her assistance with arthropod identification.
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