Semester of Graduation

Fall 2023


Master of Science (MS)


School of Renewable Natural Resources

Document Type



Successful conservation of Neotropical migratory passerines requires a detailed understanding of their annual cycle ecology, including wintering and breeding distributions, migratory pathways and stopovers, and migratory phenology. It is also important to evaluate the extent of geographic isolation between separate breeding populations during the nonbreeding season, known as migratory connectivity. Swainson’s Warbler (Limnothlypis swainsonii) is a species of conservation concern that, due to their secretive nature and selection of dense forest habitats, is poorly studied and lacks migratory information. Our objective was to track Swainson’s Warblers across their breeding populations (Mississippi Valley, Coastal Plains, and Appalachian) to detail wintering distributions, migratory routes, migratory behavior, and subsequently determine the degree of migratory connectivity with the use of barometric and light-level geolocators. Historically, tracking small passerines through migration has provided coarse and incomplete information. Recently, data collected by barometric geolocators (‘pressure tags’) that record atmospheric pressure have been combined with global weather reanalysis and a movement model to estimate precise, continuous locations, as well as collect novel flight behavior data.

In collaboration with other researchers, we captured male Swainson’s Warblers on breeding territories in 2021 and deployed 87 geolocators across six states. We recaptured 31 (36%) tagged birds in 2022. Three tags from the Great Dismal Swamp were stolen in transit, and two tags malfunctioned, leaving 26 for analysis. Light-level geolocators (n = 12) experienced chronic shading, resulting in broad wintering estimates and little information during migration. However, pressure tags (n = 14) provided accurate and refined wintering estimates (~10 – 40 km accuracy), migratory stopover regions, detailed flight data, and exact movement timestamps. We found a longitudinal divide in wintering distributions: western breeders of the Mississippi Alluvial Valley flew across the Gulf of Mexico and wintered throughout the Yucatan; eastern breeders from both the Appalachian and Coastal Plains wintered in Cuba as well as the Bahamas, with en route stops in Florida and Georgia. Surprisingly, birds traveled significantly faster and spent less time at stopovers during fall migration than in spring, antithetical to current understanding. Average and maximum flight altitudes were positively correlated with flight distance, and birds departed earlier at night for longer flights while short-distance flights had highly variable departure times. We also observed interesting flight behavior such as rapid ascents at the beginning of flights, extreme fluctuations in altitudes during flight, and short (1-2 hour) flights towards the end of spring migration. Our study provides novel insight into Swainson’s Warbler migration ecology and identifies high intra-population connectivity and moderate inter-population connectivity across their wintering grounds.

By using both barometric and light-level geolocators, our study demonstrated the multiple advantages pressure tags have over other tracking technologies for studying small birds. The development of the GeoPressureR package now allows researchers to generate accurate and continuous location estimates that would otherwise be unattainable from archival GPS tags, automated radio telemetry, and light-level geolocators. Detailed flight schedules readily delineate exact departure, arrival, and stopover periods, which can greatly improve our understanding of migration phenology. Furthermore, the visualization of flight altitudes allows us to uncover behaviors and strategies birds use to complete their migratory journey or even daily elevational movements. A global deployment of pressure tags has the potential to greatly increase our understanding of bird migration and connectivity, as well as revolutionize the way we study migration ecology and behavior



Committee Chair

Philip Stouffer