NEST SURVIVAL, FLEDGLING HABITAT USE, AND SONG STRUCTURE IN THE BLACK-CAPPED VIREO (VIREO ATRICAPILLA)

Aurora Schermbeck

The black-capped vireo (Vireo atricapilla, BCVI) is a Neotropical migratory songbird of the Vireonidae family. Over the past century, the black-capped vireo has suffered large population losses due to habitat loss and nest parasitism and was listed as endangered in 1987. Extensive recovery efforts have allowed populations to stabilize, and in 2018 the species was downlisted to ‘near-threatened’, however it is now considered a conservation reliant species, meaning that its habitat must be actively managed to remain viable. In addition, due to the black-capped vireo’s limited range and historically small populations, the species remains generally understudied.In 2020, a new black-capped vireo population was identified at the Lowder Tract site within Government Canyon State Natural Area (GCSNA) outside San Antonio, TX. The Lowder Tract site is currently made up of the shrubby, mid-successional habitat black-capped vireos use for nesting, however management of the site’s vegetation is necessary for it to remain suitable habitat. Little is currently known about the specific vegetation characteristics used by this population, but such information is vital for effective management. Thus, this project aims to discover more about both the GCSNA population and black-capped vireo behaviors as a whole by investigating three questions: 1) Do nest site vegetation characteristics impact black-capped vireo nest survival and clutch size?, 2) What vegetation characteristics are used by fledgling black-capped vireos, and do fledglings move towards specific landscape features?, and 3) Do body morphometrics and territory quality traits predict song structure in male black-capped vireos? To address these questions, we banded a focal population of 59 males between April and July of 2022-2025. For each male we attempted to record morphometrics, established territory size, and conducted nest searches. For focal males in 2024 and 2025, we also recorded at least 50 songs. We radiotagged the largest nestling from discovered nests on day 7 and tracked all tagged individuals for 14 days after fledging. We also conducted vegetation surveys at each nest site and at fledgling locations on day 7 after fledging. We found that nest survival positively covaried with vegetation height directly surrounding the nest, as well as the percentage of trees and shrubs at the nest site, while clutch size positively covaried with percentage of shrubs, and negatively covaried with nest visibility. In addition, fledglings in this population appear to move slightly toward riparian vegetation, and away from old growth juniper forest, and tend to select for areas of higher canopy cover. We also found that while song traits appear to convey little measurable information about male condition, they are an indicator of resource holding potential, with males in higher quality territories also singing higher quality song. Overall, these results emphasize the importance of vegetation characteristics on multiple aspects of black-capped vireo ecology. Thus, we intend to use the results of this investigation to aid Government Canyon staff in modifying their adaptive management plan to encourage population growth and sustainability, as well as better understand the behavior and life history of this understudied songbird.

NEST SURVIVAL, FLEDGLING HABITAT USE, AND SONG STRUCTURE IN THE BLACK-CAPPED VIREO (VIREO ATRICAPILLA)

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