High-Latitude Passerine Migrants Overlap Energetically Demanding Events in Autumn

Abstract

ABSTRACT Temporal constraints on migratory birds to molt, store fat, and migrate in autumn are probably most severe in populations breeding at high latitudes. We examined whether high-latitude time constraints were related to the overlap of these energetically demanding events in migratory passerine species. We also examined how much overlap of molt and fattening occurs within individuals. Data were collected on molt intensity and subcutaneous fat during autumn migration from 1992 to 2004 in Fairbanks, Alaska, (64° 50′ N 147° 50′ W). Among 17 migrant species, we found a negative relationship between length of breeding ground occupancy (number of days between median spring and autumn passage, our measure of time constraints) and the amounts of molt-migration overlap. There was also a positive relationship between molt-fat overlap and distance to wintering range among these 17 species. No individual completely overlapped the peak levels of both molt intensity and fat storage observed within a species, but s...