Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 97-103). Also available via the Internet.

This study compared male response to playback and interspecific vocal imitation in two White-eyed Vireo populations, in southwestern Virginia and eastern North Carolina. White-eyed Vireos are hypothesized to imitate notes from other birds which transmit efficiently through dense habitat. Male White-eyed Vireos did not respond differently to playback of each other I s songs. The song repertoires between Virginia and North Carolina were not found to include strikingly different imitations despite the different sound environments. Degradation and attenuation of White-eyed Vireo song notes was also compared between the two locations. There was no statistically significant difference in degradation or attenuation of notes when compared to native or foreign habitat. Notes occurring more frequently in the population did not transmit more efficiently. Notes unique to North Carolina did transmit more efficiently in their native habitat, but this difference was explained by the relatively less dense habitat in North Carolina. / Master of Science

Predation is the leading cause of nest failure in songbirds. My study identified nest predators of black-capped vireos and white-eyed vireos, quantified the activity of potential predator species, examined the relationships between vegetation and nest predators, and examined the relationship between nest predation and parasitism by brown-headed cowbirds. In 2008 and 2009 I monitored black-capped and white-eyed vireo nests on privately-owned properties in Coryell County and black-capped vireo nests on Kerr WMA in Kerr County and at Devils River State Natural Area in Val Verde County (2009 only). I monitored vireo nests using a video camera system to identify predators and nest fate. I also collected at-nest vegetation measurements including nest height, distance to nearest habitat edge, and nest concealment. Additionally, I sampled potential predator activity at a subset of black-capped vireo and white-eyed vireo nests in Coryell County using camera-trap bait stations and herptofaunal traps. I monitored 117 black-capped vireo nests and 54 white-eyed vireo nests. Forty-two percent of black-capped vireo and 35% of white-eyed vireo nests failed due to predation. I recorded >10 total predator species and 37 black-capped vireo and 15 white-eyed vireo nest predation events. Snakes (35%) and cowbirds (29%) were the most frequently identified nest predators; however, major predator species varied by location. I observed no significant relationship between nest fate (fledge vs. fail) and nest concealment or distance to edge for either vireo species. Nest height, concealment and distance to edge may relate to predator species in Coryell Co. for snake species, and Kerr for avian species. Additionally, I observed no difference between the predator activity and the fate of the nest. Both vireos have multiple nest predator species. Additionally, multiple cowbird predations demonstrate this species may have multi-level impacts on vireo productivity, even with active cowbird management. Vegetation structure and concealment may also affect predator species. However, the activity of other predator species near active nests may not negatively affect nest success.

Brown-headed cowbird (Molothrus ater) parasitism on the endangered black-capped vireo (Vireo atricapilla) is a management concern. Management for black-capped vireos in east-central Texas has focused on habitat restoration through vegetation modification. Loss of productivity due to parasitism, however, may limit the potential habitat restoration in restoring black-capped vireo populations. I investigated the relationships between frequency of parasitism on black-capped and white-eyed (V. griseus) vireos and characteristics of the neighboring avian assemblage to identify avian characteristics useful for predicting expected frequency of parasitism. I located and monitored vireo nests in March–July 2006 to determine frequency of cowbird parasitism on the 2 vireo species on 6 study sites on private land in east-central Texas. I conducted 100-m-radius circular-plot point count surveys on study sites from 1 April– 31 June 2006 to identify neighboring avian assemblages. All black-capped vireo nests (n = 20) and 81% of white-eyed vireo nests (n = 27) were parasitized. Species richness, species evenness, cumulative bird abundance, and cowbird host abundance did not differ significantly among study sites. Parasitism frequency was positively correlated with the number of cowbirds detected (r2 = 0.673, P= 0.045). Parasitism frequency was negatively correlated with abundance (r2 = 0.687, P = 0.042) and proportion of (r2 = 0.692, P = 0.040) of black-and-white warblers (Mniotilta varia), known cowbird hosts. I did not find a relationship between parasitism frequency and abundance of other cowbird hosts. The relationship between black-andwhite warbler abundance and parasitism frequency may reflect an interaction between parasitism and the vegetation characteristics associated with black-and-white warblers, suggesting the need for future study of the relationships between parasitism frequency and vegetation characteristics in the study region. The frequency of parasitism I observed suggests that cowbird control may be an important management tool if management objectives in the study region continue to include recovery of black-capped vireo populations. Number of cowbirds may be a reasonable predictor of expected parasitism frequency in an area, which may be useful to land managers in selecting areas for implementation of cowbird control.

Preservation of large tracts of habitat is often recommended for long-term population viability of area-sensitive species. Large tracts may not always be available. Smaller patches, though not able to contain a viable population individually, may contribute to overall regional population viability if within the small patches pairs could successfully reproduce. By definition, area-sensitive species should have a minimum patch size threshold of habitat below which they will not likely reproduce. Two potential causes for positive relationships between patch size and production are inverse relationships between patch size and brood parasitism and patch size and food availability. My objectives were (1) to determine the minimum patch size thresholds of reproductive success for golden-cheeked warblers (Dendroica chrysoparia), black-and-white warblers (Mniotilta varia), and white-eyed vireos (Vireo griseus); (2) to determine whether thresholds for occupancy, territory establishment by males, or pairing success were indicative of thresholds of reproduction; (3) to determine whether the proportion of pairs fledging brown-headed cowbird (Molothrus ater) young was related to patch size, and (4) to determine the affects of patch size on food availability (i.e., arthropod abundance). The Vickery index of reproductive activity was used to determine reproductive activity of each male or pair and to quantify parasitism occurrences. I collected arthropods using branch clipping to assess the relationship between patch size and arthropod abundance. I found minimum patch size thresholds of reproductive success for golden-cheeked and black-and-white warblers, but not for white-eyed vireos. Minimum patch size of reproductive success was between 15 and 20.1 ha. Minimum patch size thresholds for occupancy, territory establishment by males, and pair formation were not consistent with thresholds for reproductive success. I found no relationships between patch size and cowbird parasitism or patch size and arthropod biomass. Conservation practices for target species based on thresholds of occupancy, territory establishment, or pair formation may not address issues of reproduction. The ability to identify thresholds of reproductive success for target species could be useful in conservation and management in multiple ways including setting goals for retention and restoration of a target species’ habitat patch size.

Preservation of large tracts of habitat is often recommended for long-term population viability of area-sensitive species. Large tracts may not always be available. Smaller patches, though not able to contain a viable population individually, may contribute to overall regional population viability if within the small patches pairs could successfully reproduce. By definition, area-sensitive species should have a minimum patch size threshold of habitat below which they will not likely reproduce. Two potential causes for positive relationships between patch size and production are inverse relationships between patch size and brood parasitism and patch size and food availability. My objectives were (1) to determine the minimum patch size thresholds of reproductive success for golden-cheeked warblers (Dendroica chrysoparia), black-and-white warblers (Mniotilta varia), and white-eyed vireos (Vireo griseus); (2) to determine whether thresholds for occupancy, territory establishment by males, or pairing success were indicative of thresholds of reproduction; (3) to determine whether the proportion of pairs fledging brown-headed cowbird (Molothrus ater) young was related to patch size, and (4) to determine the affects of patch size on food availability (i.e., arthropod abundance). The Vickery index of reproductive activity was used to determine reproductive activity of each male or pair and to quantify parasitism occurrences. I collected arthropods using branch clipping to assess the relationship between patch size and arthropod abundance. I found minimum patch size thresholds of reproductive success for golden-cheeked and black-and-white warblers, but not for white-eyed vireos. Minimum patch size of reproductive success was between 15 and 20.1 ha. Minimum patch size thresholds for occupancy, territory establishment by males, and pair formation were not consistent with thresholds for reproductive success. I found no relationships between patch size and cowbird parasitism or patch size and arthropod biomass. Conservation practices for target species based on thresholds of occupancy, territory establishment, or pair formation may not address issues of reproduction. The ability to identify thresholds of reproductive success for target species could be useful in conservation and management in multiple ways including setting goals for retention and restoration of a target species’ habitat patch size.