Defining catchment origins of a geographical bottleneck: Implications of population mixing and phenological overlap for the conservation of Neotropical migratory birds

Migratory bottlenecks concentrate individuals and populations of Nearctic-Neotropical migrants from across vast breeding areas. The extent to which such concentrations occur has important ramifications for interpreting migratory connectivity and for the vulnerability of populations throughout the annual cycle but investigations of such phenomena are rare. We inferred breeding origins of 11 species of Nearctic-Neotropical migrants captured during fall migration in the Darien region, northwestern Colombia, using feather stable-hydrogen isotope values (delta H-2(f)). Our objectives were to determine (1) the extent to which this region acts as a catchment for individuals from across a species' breeding range, (2) if temporal patterns of arrival arise as a function of migration distance inferred from delta H-2(f), and (3) if phenological differences among species segregate populations. The Darien concentrated populations of 6 species, with individuals potentially derived from 78% to 94% of their respective North American breeding ranges. The catchment area for the remaining 5 species covered 40-72% of breeding ranges, with a bias in origin from the west (e.g., Northern Waterthrush [Parkesia noveboracensis], Swainson's Thrush [Catharus ustulatus]), north or center (Yellow Warbler [Setophaga petechia]) of their range. Differential timing in the migration of populations occurred in 6 species, generating a degree of temporal segregation. In contrast, peak migration for all species occurred in October, resulting in high overlap among species in their use of the Darien. Overall, our results describe high mixing of populations in the Darien for several species, which, given the region's highly restricted geographic extent, suggests that a bottleneck effect occurs; however, comparisons with other regions in Central America and northern South America are required. The combination of extensive population mixing and limited temporal segregation over a narrow geographical area suggests that long-distance migrants may be especially vulnerable to events occurring in such regions, including adverse weather conditions and anthropogenic land cover change.