Integrating information from geolocators, weather radar, and citizen science to uncover a key stopover area of an aerial insectivore

Determining the distribution of stopover and overwintering areas of migratory animals is essential for understanding population dynamics and building predictive models. Tree Swallows (Tachycineta bicolor) are small songbirds that breed across North America. Data from Doppler weather radar and eBird indicate that Tree Swallow numbers increase throughout October and November in southeastern Louisiana, but then decrease during December. We thus hypothesized that southeastern Louisiana is a stopover area used by Tree Swallows during fall migration before they move to farther overwintering areas. We tested this hypothesis by attaching light-logging geolocators to Tree Swallows at five breeding sites spanning the species' breeding range from British Columbia to Nova Scotia, and then tracking their fall migration routes, stopover sites, and wintering locations. Of 38 individuals that returned in the following breeding season, 11 birds from three breeding sites (Saskatchewan, Wisconsin, and Ontario) used southeastern Louisiana as a stopover site. Arrival date and duration of stay closely matched observations from both eBird and radar data. From Louisiana, most Tree Swallows continued their migration to one of three wintering sites: peninsular Florida, the Bahamas, or the Yucatán Peninsula, whereas two birds remained until spring within 200 km of the stopover area. Our results (1) suggest that southeastern Louisiana is an extended stopover site for Tree Swallows that originate from a wide geographic range on the breeding grounds; and (2) demonstrate how geolocators, combined with other sources of movement information, reveal habitat use throughout the annual cycle.