Rangewide connectivity and migration ecology in Limnothlypis swainsonii (Swainson’s Warbler) uncovered with barometric geolocators

Abstract Conserving migratory birds requires a detailed understanding of their full annual cycle, including nonbreeding distributions, migratory pathways, migratory phenology, and patterns of migratory connectivity. This information could be particularly important for managing Limnothlypis swainsonii (Swainson’s Warbler), a Nearctic–Neotropical migrant of conservation concern. We tracked L. swainsonii from across their breeding distribution in the southeastern U.S. to identify migration routes and nonbreeding locations. In addition to traditional light-level geolocators, we used barometric pressure data collected by multisensor geolocators (pressure tags), which we combined with a global weather reanalysis and a movement model (GeoPressureR) to estimate precise, continuous locations, and collect previously unavailable flight behavior data. We deployed 43 pressure tags and 43 light-level geolocators on breeding male L. swainsonii across 6 states in 2021. We recaptured 31 (36%) tagged individuals in 2022, with 26 tags used for analysis. Light-level geolocators (n = 12) experienced chronic shading, resulting in broad nonbreeding estimates and little information during migration. Pressure tags (n = 14), however, provided accurate nonbreeding estimates (∼10–40 km) and all stopover regions. We found a longitudinal divide in nonbreeding distributions: western breeders of the Mississippi Valley and Missouri Ozarks migrated across the Gulf of Mexico to spend the nonbreeding season in the Yucatan Peninsula; eastern breeders from the Appalachians and Atlantic Coastal Plains migrated to Cuba and the Bahamas, with en route stops in Florida and Georgia. Pressure tags also revealed precise details of migratory movements, including phenology, flight durations and altitudes. Most notably, individuals traveled significantly faster and spent less time at stopovers during fall migration than in spring, antithetical to current understanding. Migratory pathways and wintering grounds of L. swainsonii suggest eastern and western breeding populations experience different environmental conditions, ecological and anthropogenic barriers, and habitat quality during migration.