Evidence of widespread movements from breeding to molting grounds by North American landbirds

Precise locations for the complete feather molt of North American landbirds, relative to their breeding territories, remain undocumented for most species. We analyzed >760,000 records of 140 species at 936 bird-capture stations to assess probabilities of recording both molting landbirds at their breeding sites and breeding birds at molting sites, and to investigate latitudinal, longitudinal, and elevational shifts from breeding to molting grounds. We demonstrate widespread evidence for molt-migrations among a variety of North American landbirds, including many migratory species previously thought to molt on "breeding grounds." Geospatial differences between breeding and molting grounds were detected in all 4 compass directions as well as both upslope and downslope in elevation, while individuals of some species appeared to disperse to specific molting locations not discriminated by spatial direction or elevation from breeding territories. Although western North American species and populations are reported to undergo more molt-migration than eastern species, our molt-movement probabilities were similar in western and eastern North America and were greater in the east than in the west for several species. Combining our results with those of these previous studies, we suggest that many landbird species in western North America move longer distances to molt, whereas many landbirds in eastern North America may equally or more likely move shorter distances between breeding and molting habitats, while remaining within the overall breeding ranges of the species. Heterogeneous molt-movement responses are suggested for many species, which may relate to breeding success, resource availability on breeding territories, weather events, and other factors. Researchers using stable-isotopic, genetic, and geolocator techniques need to consider potential molt-movement strategies, and to incorporate molting habitat requirements into full-annual-cycle conservation efforts.