Breeding destinations and spring migration patterns of Swainson's Thrush (Catharus ustulatus) at a Costa Rican stopover site

摘要

Stable-isotope analysis of feathers has become a useful tool for examining migration patterns and annual connectivity of migratory songbird populations. We used this approach, combined with molecular sex-identification, to examine expected breeding destinations and migration ecology of Swainson's Thrush (Catharus ustulatus) at a spring stopover site on the Osa Peninsula of Costa Rica. The number of individuals passing through the site increased in late March and peaked in mid-April before declining toward late April. Plumage and expected feather-deuterium (delta D-f) values for North America indicated that most individuals had probably bred or fledged in west-central regions of the boreal forest and western mountains, rather than farther north in the Yukon and Alaska or in eastern regions of Canada and the United States. Average delta D-f values differed little between males and females, indicating similar breeding destinations, though females tended to show greater variability. The delta D-f values of second-year and after-second-year birds differed only slightly, which suggests that most adults molted near the breeding grounds. Timing of migration through the site was not related to breeding latitude as inferred from feather delta D. We also found little relation between the energetic reserves carried by individuals and their expected breeding destination, possibly because the reserves carried at this stage have little relation to the total reserves needed by northern breeders for the final stage of migration.