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 several individuals approached this theoretical maximum in four species. Molt-fat overlap was highest in an individual Yellow Warbler (Setophaga petechia) that achieved 70% of the maximum possible overlap of peak fat storage and peak molt intensity for that species. These findings indicate that high-latitude passerines can overlap energetically demanding events during the annual cycle but that there is considerable variation among species in how they juggle time and energy constraints. Our data provide strong support for a conceptual model that passerine migrants breeding at high latitudes use strategies that reduce the time required to complete breeding season activities. In doing so, many of these birds appear to push energetic limits by overlapping molt, migration, and fattening to a degree not previously documented.
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