Temporal patterns of migration, molt, and fat storage among high-latitude passerine migrants

Abstract

Aspects of migration, fattening, and molt in trans-continental passerine migrants were examined during spring and autumn migration in Fairbanks, Alaska (64°50’ N, 147°50’ W). From 1992-1998, 25,718 birds of 18 species were banded. Based on median dates of spring and autumn passage, species-level estimates of the duration of breeding range occupation ranged from 48 to 129 days. Adults departed significantly later than immatures in 11 of the 18 species examined and significantly earlier than immatures in only one species, the Alder Flycatcher. Adults had significantly higher fat scores than immatures in most species, but these differences were attributable to the influence of ambient temperatures, length of preceding night, and the time of day the bird was captured. Adults of many species overlapped the final stages of the prebasic molt with autumn migration, and individuals that did so had less stored fat than those that were not molting.