MIGRATION ECOLOGY OF NEOTROPICAL MIGRANT SONGBIRDS EN ROUTE THROUGH THE DARIÉN REGION OF COLOMBIA

Abstract

Neotropical migrant songbirds have experienced severe and ongoing population declines. The lack of knowledge about migration outside North America inspired me to study the ecology of migration of a group of 11 species to determine: 1) breeding origins and patterns of migration, 2) how breeding origins, age, sex, El Niño–Southern Oscillation (ENSO), body size and route of migration affect body condition and phenology. I also described migration in detail for Canada Warbler due to its status as a species of concern. Constant-effort mist netting data and samples were collected for stable isotope (δ2H) analyses to determine breeding origin. For the 11 study species, we found populations derived from across their breeding range in 55% of species, while 27% had more westerly origins and 18% from eastern portions of their breeding range. The peak of migration for all species, except Yellow Warbler (late September), occurred in October. I found evidence for chain migration in 36% of species (Red eyed vireo, Yellow Warbler, Northern Waterthrush and Mourning Warbler) and leapfrog migration in 18% (Swainson’s thrush and Gray-cheeked Thrush). Body condition was little affected by factors such as breeding origin, the ENSO, age and sex and route of migration. In contrast, body size affected almost half of the species (Veery, Swainson’s Thrush, Gray-cheeked Thrush, Scarlett Tanager and Summer Tanager). For Canada Warbler, migration through Central America rather than across the Caribbean Sea was most probable and the study site was more frequently used by hatch-year birds (91% vs. 9% after hatch-year birds), and females (67% vs. 33% males). Most Canada Warblers (89%) arrived with low fuel reserves. These results combined with estimated flight ranges revealed that 46% of Canada Warblers captured in the Darién likely needed to iii refuel to continue migrating. The large concentration of Neotropical migrants from across North America stopping in the Darién site confirms the ecological function of the Darién where potentially a large proportion of the breeding population of several species refuels or rests before reaching their wintering sites in South America.