Regular issues
Monographs
Special Issues



Southeastern Naturalist
    SENA Home
    Range and Scope
    Board of Editors
    Staff
    Editorial Workflow
    Publication Charges
    Subscriptions

Other EH Journals
    Northeastern Naturalist
    Caribbean Naturalist
    Neotropical Naturalist
    Urban Naturalist
    Eastern Paleontologist
    Journal of the North Atlantic
    Eastern Biologist

EH Natural History Home

Tree Species Use and Seasonal Response to Food Availability of Black-Capped Vireo
David T. Morgan, M. Clay Green, Michael L. Morrison, and Thomas R. Simpson

Southeastern Naturalist, Volume 17, Issue 4 (2018): 560–582

Full-text pdf (Accessible only to subscribers.To subscribe click here.)

 

Site by Bennett Web & Design Co.
Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 560 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 Southeastern Naturalist 561 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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. Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 562 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 Southeastern Naturalist 563 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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 Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 564 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 Southeastern Naturalist 565 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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). Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 566 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 Southeastern Naturalist 567 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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 Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 568 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 Southeastern Naturalist 569 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 (± 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. Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 570 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 Southeastern Naturalist 571 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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. Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 572 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, Southeastern Naturalist 573 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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 Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 574 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 Southeastern Naturalist 575 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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 Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 576 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, Southeastern Naturalist 577 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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 Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 578 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. Southeastern Naturalist 579 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 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. Literature Cited Bailey, J.W. 2005. Hierarchical nest-site selection and the effects of habitat characteristics on Black-capped Vireo nest survival. M.Sc. Thesis. University of Missouri, Columbia, MO. Barr, K.R., D.L. Lindsay, G. Athrey, R.F. Lance, T.J. Hayden, S.A. Tweddale, and P.L. Leberg. 2008. Population structure in an endangered songbird: Maintenance of genetic differentiation despite high vagility and significant population recovery. Molecular Ecology 16:3628–39. Beal, F.E.L. 1907. Birds of California, in Relation to the Fruit Industry, Part 1. US Department of Agriculture, US Government Printing Office. Washington, DC. 96 pp. Benson, R.H., and K.L.P. Benson. 1990. Estimated size of Black-capped Vireo population in northern Coahuila, Mexico. The Condor 92:777–779. Bibbey, C.J., N.D. Burges, D.A. Hill, and S.H. Mustoe. 2000. Bird-Census Techniques, 2nd Edition. Academic Press, San Diego, CA. 302 pp. Campbell, L. 2003. Endangered and Threatened Animals of Texas: Their Life History and Management. Texas Parks and Wildlife Department, Austin, TX. 127 pp. Chapin, E.A. 1925. Food habits of the vireos: A family of insectivorous birds. Department Bulletin 1355. US Department of Agriculture. Washington, DC. Cimprich, D.A., and R.M. Kostecke. 2006. Distribution of the Black-capped Vireo at Fort Hood, Texas. Southwestern Association of Naturalists 51:99–102. Cooksey, M.L., and J.E. Thompson. 2005. Monitoring the Golden-cheeked Warbler and the Black-capped Vireo on Camp Bullis Texas: 2005 field season report. Unpublished Report, Essex Corporation, Fort Sam Houston , TX. Cooper, R.J., and R.C. Whitmore. 1990. Arthropod sampling methods in ornithology. Studies in Avian Biology 13:29–37. Cooper, R.J., P.J. Martinat, and R.C. Whitmore. 1990. Dietary similarity among insectivorous birds: Influence of taxonomic versus ecological categorization of prey. Studies in Avian Biology 13:104–109. Dodge, K.M., R.C. Whitmore, and E.J. Harner. 1990. Analyzing foraging use and availability using regression techniques. Studies in Avian Biology 13:318–324. Dufault, D. 2004. Habitat occupancy by the Black-capped Vireo (Vireo atricapilla) following prescribed burns at Kerr Wildlife Management Area. M.Sc. Thesis. Texas State University, San Marcos, TX. Eaton, E.R., and K. Kaufman. 2007. Kaufman Field Guide to Insects of North America. Houghton Mifflin Company, New York, NY. 392 pp. Eckrich, G.H., T.E. Koloszar, and M.D. Goering. 1999. Effective landscape management of Brown-headed Cowbirds at Fort Hood, Texas. Studies in Avian Biology 18:267–274. Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 580 Environmental Systems Research Institute (ESRI). 2009. ARCVIEW GIS. Version 9.3.1 Environmental Systems Research Institute, Redlands, CA. Farquhar, C.C., J.I. Gonzalez, M.A. Guerrero, and O. Ballesteros. 2003. Breeding habitat, distribution, and population status of the Black-capped Vireo in Northern Mexico. Interim Report, Section 6 Grant No. E-17. Wildlife Diversity Program, Texas Parks and Wildlife Department, Austin, TX. Fazio, V.W., III. 1994. Genetic variation in an endangered passerine, Vireo atricapillus Woodhouse. M.Sc. Thesis. Ohio University, Athens, OH. Fazio, V.W., III, D.B. Miles, and M.W. White. 2004. Genetic differentiation in the endangered Black-capped Vireo. The Condor 106:377–385. Gabbe, A.P., S.K. Robinson, and J.D. Brawn. 2002. Tree-species preferences of foraging insectivorous birds: Implications for floodplain forest restoration. Conservation Biology 16:462–470. González-Rojas J.I., C.C. Farquhar, M. Guerrero-Madriles, O. Ballesteros-Medrano, and A. Núñez-Gonzalí. 2014. Breeding records of Black-capped Vireo (Vireo atricapilla) in Northeastern Mexico. Wilson Journal of Ornithology 126:151–155. Graber, J.W. 1961. Distribution, habitat requirements, and life history of the Black-capped Vireo (Vireo atricapilla). Ecological Monographs 31:313–336. Greenman, T.M. 1995. Postfire characteristics of existing and potential Black-capped Vireo (Vireo atricapillus) nesting habitat on the Wichita Mountains Wildlife Refuge, Oklahoma. M.Sc. Thesis. Oklahoma State University, Stillwater, OK. Grzybowski, J.A. 1995. Black-capped Vireo (Vireo atricapilla). No. 181, In A.F. Poole and F.B. Gill (Eds.). The Birds of North America online. Cornell Lab of Ornithology, Ithaca, NY. Available online at https://birdsna.org/Species-Account/bna/species/bkcvir1/introduction. Accessed 2 February 2010. Grzybowski, J.A., D.J. Tazik, and G.D. Schnell. 1994. Regional analysis of Black-capped Vireo breeding habitats. The Condor 96:512–544. Guilfoyle, M.P. 2002. Black-capped Vireo and Golden-cheeked Warbler populations potentially impacted by USACE reservoir operations. EMRRP Technical Notes Collection (TN EMRRP-SI-28), US Army Engineer Research and Development Center, Vicksburg, MS. Hejl, S.J., and J. Verner. 1990. Within-season and yearly variations in avian foraging locations. Studies in Avian Biology 13:202–209. Holmes, R.T., and S.K. Robinson. 1981. Preferences of foraging insectivorous birds in a northern hardwoods forest. Oecologia 48:31–35. Houston, D.R. 2008. Foraging behavior of the Black-capped Vireo in central Texas. M.Sc. Thesis. Texas State University, San Marcos, TX. Hutto, R.L. 1981. Temporal patterns of foraging activity in some wood warblers in relation to the availability of insect prey. Behavioral Ecology and Sociobiology 9:195–198. Hutto, R.L. 1985. Seasonal changes in the habitat distribution of transient insectivorous birds in south-eastern Arizona: Competition mediated? Auk 102:120–132. Hutto, R.L. 1990. Studies of foraging behavior: Central to understanding the ecological consequences of variation in food abundance. Studies in Avian Biology 13:389–390. Johnson, M.D., and T.W. Sherry. 2001. Effects of food availability on the distribution of migratory warblers among habitats in Jamaica. Journal of Animal Ecology. 70:546–560. Karr, J.R., and J.D Brawn. 1990. Food resources of understory birds in central Panama: Quantification and effects on avian populations. Studies in Avian Biology 13:58–64. Southeastern Naturalist 581 D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 Kendeigh, S.C. 1970. Energy requirements for existence in relation to body size. The Condor 72:60–65. Kostecke, R.M., S.G. Summers, G.H. Eckrich, and D.A. Cimprich. 2005. Effects of Brownheaded Cowbird (Molothrus ater) removal on Black-capped Vireo (Vireo atricapilla) nest success and population growth at Fort Hood, Texas. Ornithological Monographs 57:28–37. Lehman, R.D. 1982. Mites (Acari) of Pennsylvania conifers. Transactions of the American Entomological Society 108:181–286. Maresh, J. 2005. Monitoring construction activities adjacent to endangered Black-capped Vireo habitat on Buffalo Gap wind-energy project, Taylor County, Texas. Turner Biological Consulting, LLC, prepared for SeaWest WindPower, Inc., Buffalo Gap, TX. Marshall, M.E., Morrison, M.L., and R.N. Wilkins. 2013. Tree species composition and food availability affect productivity of an endangered species. The Condor 115(4):882–892. McGrath, L.J., C.V. Riper III, and J.J. Fontaine. 2008. Flower power: Tree flowering phenology as a settlement cue for migrating birds. Journal of Animal Ecology 78:22–30. Morgan, D.T. 2012. Foraging ecology and forage availability for the Black-capped Vireo. M.Sc. Thesis. Texas State University. San Marcos, TX. Morrison, M.L. 1984. Influence of sample size and sampling design on analysis of avian foraging behavior. Condor 86:146–150. Noa, L.A., D.H. Hirth, T.M. Donovan, and D. Cimprich. 2007. Demographic differences of Black-capped Vireos in 2 habitat types in central Texas. Journal of Wildlife Management 71:1042–1049. Nolan, V., Jr., and D.P. Wooldridge. 1962. Food habits and feeding behavior of the Whiteeyed Vireo. The Wilson Bulletin 74:68–73. Orians, G.H. 1980. Some Adaptations of Marsh-nesting Blackbirds. Princeton University Press, Princeton, NJ. 312 pp. Pinkston, J., J. Maresh, and N. Wright. 2002. Population monitoring for the Black-capped Vireo (Vireo atricapillus) at Fossil Rim Wildlife Center, Dinosaur Valley State Park, and adjacent private property in Somervell County, Texas (2001 Field Season). Final Report, Texas Parks and Wildlife Department, Austin, TX. Pyke, G.H., H.R. Pulliam, and E.L. Charnov. 1977. Optimal foraging: A selective review of theory and tests. The Quarterly Review of Biology 52:137–154. Quinn, M.A. 2000. Abundance and distribution of potential arthropod prey species in a typical Golden-cheeked Warbler habitat. M.Sc. Thesis. Texas A&M University, College Station, TX. Ratzlaff, A. 1987. Endangered and threatened wildlife and plants: Determination of the Black-capped Vireo to be an endangered species. Federal Register 52:37420-374 23. Remsen, J.V., and S.K. Robinson. 1990. A classification scheme for foraging behavior of birds in terrestrial habitats. Studies in Avian Biology 13:144–160. Robinson, S.K., and R.T. Holmes. 1982. Foraging behavior of forest birds: The relationships among search tactics, diet, and habitat structure. Ecolog y 63:1918–1931. Rohwer, S., and C.D. Spaw. 1988. Evolutionary lag versus bill-size constraints: A comparative study of the acceptance of cowbird eggs by old hosts. Evolutionary Ecology 2:27–36. Sexton, C.W. 2002. List of Black-capped Vireo populations on Balcones Canyonlands NWR. Unpublished manuscript. US Fish and Wildlife Service, Marble Falls, TX. Sexton, C.W. 2005. Black-capped Vireo populations on Balcones Canyonlands National Wildlife Refuge. Unpublished data. US Fish and Wildlife Service, Marble Falls, TX. Southeastern Naturalist D.T. Morgan, M.C. Green, M.L. Morrison, and T.R. Simpson 2018 Vol. 17, No. 4 582 Sherry, T.W., and L.A. McDade. 1982. Prey selection and handling in two neotropical hover-gleaning birds. Ecology 63:1016–1028. Sokal, R.R., and F.J. Rohlf. 2012. Biometry, 4th Edition. W.H. Freeman, New York, NY. 937 pp. Stake, M.M., and D.A. Cimprich. 2003. Using video to monitor predation at Black-capped Vireo nests. The Condor 105:348–357. Tazik, D.J., J.A. Grzybowski, and J.D. Comelius. 1993. Status of the Black-capped Vireo at Fort Hood, Texas, Volume II: Habitat. US Army Corps of Engineers, Construction Engineering Research Laboratories, Champaign, IL. Thomas, D.L., and E.J. Taylor. Study designs and tests for comparing resource use and availability. II. Journal of Wildlife Management 70:324–336. US Fish and Wildlife Service (USFWS). 1991. Black-capped Vireo recovery plan. Albuquerque, NM. USFWS. 1992. Golden-cheeked Warbler recovery plan. Albuquerque, NM. USFWS. 2001. Balcones Canyonlands National Wildlife Refuge: Comprehensive Conservation Plan 2001–2016. Albuquerque, NM. Wagner, J.L. 1981. Visibility and bias in avian foraging data. Condor 83:263–264. Wilkins, N., R.A. Powel, T. Conkey, and A.G. Snelgrove. 2006. Population status and threat analysis for the Black-capped Vireo. Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX. Wolda, H. 1990. Food availability for an insectivore and how to measure it. Studies in Avian Biology 13:38–43. Wood, E.M., A.M. Pidgeon, F. Liu, and D.J. Mladenoff. 2012. Birds see the trees inside the forest: The potential impacts of changes in forest composition on songbirds during spring migration. Forest Ecology and Management. 280:176–186. Yard, H.K., C. van Riper III, B.T. Brown, and M.J. Kearsley. 2004. Diets of insectivorous birds along the Colorado River in the Grand Canyon, Arizona. Condor 106:106–115.