Migratory connectivity and potential nonbreeding sexual segregation in Gray Vireos (Vireo vicinior)

Nonbreeding distributions of most migratory North American breeding birds are relatively well-defined, but population- and sex-specific nonbreeding dispersion is poorly understood. Nonbreeding factors can limit population growth and manifest differently among populations and sexes across the full annual cycle. Thus, unraveling complex patterns mediating life history processes is crucial for targeted conservation of migratory animals. The Gray Vireo (Vireo vicinior) is a short-distance (i.e., ~1,000 km) migratory songbird of the southwestern United States and western Mexico for which data on migration ecology and migratory connectivity are lacking. We used light-level geolocators (48 deployed, 15 retrieved) to track migration and estimate nonbreeding locations of female and male Gray Vireos from two breeding sites in New Mexico and one breeding site in Utah, USA. Our data suggest weak-to-moderate spatial migratory connectivity (rM = 0.3), with birds from one New Mexico site wintering 600–1,000 km east of the other two populations, incongruent with geographic breeding-site dispersion. Our data also suggest that females overwintered farther north (~520 km; ~4.7° latitude; 95% CI [1.6°, 7.8°]) than males, suggesting potential sexual segregation during the stationary nonbreeding season, which may be explained by larger body size in females than males. Our results provide important details on the nonbreeding ecology of an under-studied dryland songbird, as well as suggest potential differential migration patterns, all of which warrant further study.