Timing of the Definitive Prebasic Molt in Yellow Warblers at James Bay, Ontario

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Abstract
The relationship between breeding, molt, and migration of Yellow Warblers (Dendroica petechia) was studied on the southwest coast of James Bay, Ontario from 1982 to 1984. Breeding chronology was similar in each year, with a mean fledging date of 11 to 12 July. Color-banded individuals regularly overlapped molt and care of fledglings. Known-breeding males began remigial molt 8.1 days after young fledged (range = 1-19 days), females 12.3 days (range = 9-15 days). The population mean date of molt onset was 17 July for males, and 22 July for females. Females molted significantly more rapidly than males, and both sexes completed molt on the mean data of 28 August. There were marked behavioral differences between the sexes during molt. Males typically remained on territories, advertising their presence with song. Females showed little site attachment, wandering over the study area or dispersing. Both sexes gained weight during the early and middle periods of molt, but lost weight prior to migration. Most birds began migrating during the final stages of remigial molt. Depletion of insect food and deteriorating weather in later summer appear to have selected for a rapid intense molt of 35 to 45 days, during which some birds may become virtually flightless. The importance of maintaining thermoregulatory abilities and high rates of feeding may constrain further reduction in molt duration. The need for maintaining peak flight efficiency during fledging care and migration may have partially determined the scheduling of molt in this population.