Extreme genetic similarity does not predict non-breeding distribution of two closely related warblers

Detailed knowledge of migratory connectivity can facilitate effective conservation of Neotropical migrants by helping biologists understand where and when populations may be most limited. We studied the migratory behavior and non-breeding distribution of two closely related species of conservation concern, the Golden-winged Warbler (Vermivora chrysoptera) and Blue-winged Warbler (Vermivora cyanoptera). Although both species have undergone dynamic range shifts and population changes attributed to habitat loss and social interactions promoting competition and hybridization, full life-cycle conservation planning has been limited by a lack of information about their non-breeding ecology. Because recent work has demonstrated that the two species are nearly identical genetically, we predicted that individuals from a single breeding population would have similar migratory timing and overwintering locations. In 2015, we placed light-level geolocators on 25 males of both species and hybrids in an area of breeding sympatry at the Fort Drum Military Installation in Jefferson and Lewis counties, New York. Despite extreme genetic similarity, non-breeding locations and duration of migration differed among genotypes. Golden-winged Warblers (N = 2) overwintered > 1900 km southeast of the nearest Blue-winged Warbler (N = 3) and spent nearly twice as many days in migration; hybrids (N = 2) had intermediate wintering distributions and migratory timing. Spring migration departure dates were staggered based on distance from the breeding area, and all birds arrived at the breeding site within 8 days of each other. Our results show that Golden-winged Warblers and Blue-winged Warblers in our study area retain species-specific non-breeding locations despite extreme genetic similarity, and suggest that non-breeding locations and migratory timing vary along a genetic gradient. If the migratory period is limiting for these species, our results also suggest that Golden-winged Warblers in our study population may be more vulnerable to population decline than Blue-winged Warblers because they spend almost twice as many days migrating.