1. Light-level geolocator tags use ambient light recordings to estimate the whereabouts of an individual over the time it carried the device. Over the past decade, these tags have emerged as an important tool and have been used extensively for tracking animal migrations, most commonly small birds.

1. Currently, the deployment of tracking devices is one of the most frequently used approaches to study movement ecology of birds. Recent miniaturization of lightlevel geolocators enabled studying small bird species whose migratory patterns were widely unknown. However, geolocators may reduce vital rates in tagged birds and may bias obtained movement data.2. There is a need for a thorough assessment of the potential tag effects on small birds, as previous meta-analyses did not evaluate unpublished data and impact of multiple life-history traits, focused mainly on large species and the number of published studies tagging small birds has increased substantially.

Conservation efforts directed at population declines for migratory animals must consider threats occurring at different stages often separated by vast distances. Furthermore, connectivity between populations and fidelity of individuals to specific habitats during the annual cycle are also important considerations. Avian aerial insectivores are experiencing steep population declines in North America, and those declines may be driven, in part, by conditions on the wintering grounds. Here, using geolocators (2 species; 4 individuals) and stable isotope (δ2H, δ13C and δ15N) measurements of feathers (3 species; 841 individuals), we identified approximate winter areas, and assessed migratory connectivity and among-year winter habitat fidelity for three aerial insectivores (Bank Swallow Riparia riparia, Barn Swallow Hirundo rustica and Cliff Swallow Petrochelidon pyrrhonota) that breed in northeastern North America. All three species of swallows are declining in this region. Our results, largely from the stable isotope analysis, suggest that these species likely winter throughout the Cerrado, La Plata Basin, and the Pampas, in South America. These most likely areas were similar among years (2013-2016) for Bank and Cliff Swallows, but varied for Barn Swallows (2014-2016). We found weak migratory connectivity for all three species, and, with one exception, weak habitat fidelity among years for individuals. For individual Barn Swallows captured in two or more years, we found high repeatability in δ13C values, suggesting some fidelity to similar habitats among years. The most likely wintering areas for these species coincide with large areas of South America experiencing high rates of land-use change.

No abstract

Background To effectively conserve migratory species, the entire range encompassed by their annual life cycle needs to be considered. Most research on Nearctic-Neotropical migratory birds has focused on the breeding grounds resulting in a general lack of knowledge regarding the wintering and migratory periods. The Canada Warbler (Cardellina canadensis) has declined by 71% from 1970 to 2012, at a rate of 2.9% per year, and is listed as Threatened in Canada. As with most Nearctic-Neotropical migrants, conservation efforts outside the breeding range are limited by a poor understanding of migration routes and the connectivity between specific breeding and wintering populations. Results To determine migratory routes of multiple breeding populations of Canada Warblers, we directly-tracked individuals using light-level geolocators deployed at four sites across the breeding range, spanning approximately 43 degrees in longitude (Alberta, Manitoba and Québec, Canada, and New Hampshire, USA). Twenty-five geolocators with usable data were recovered from three sites and were analyzed using FlightR to determine fall migration routes (n = 18) and individual wintering sites (n = 25). Individuals from all breeding populations took a western fall migration route at the Gulf of Mexico; with 77.8% of birds funnelling into a narrow geographic space along the western side of the Gulf of Mexico (97°W-99°W). We found no evidence for population-specific, parallel migration routes. Most individuals (72%) overwintered in Colombia. The remaining individuals overwintered in Venezuela. Conclusions Our results demonstrate convergence of migratory routes around a migration barrier for individuals originating from widely distributed breeding areas. Further, we suggest the potential importance of habitat around the Gulf of Mexico during migration and Andean forest in Colombia as overwintering habitat for this threatened species. Future research should be directed at understanding how these areas are used by Canada Warblers.