Dynamics of Postnuptial Molt in Free-Living Mountain White-Crowned Sparrows

Data were collected on postnuptial molt by frequent measurements of primaries during eight consecutive seasons on Zonotrichia leucophrys oriantha of known sex, age, and reproductive history. Molt onset, marked by the shedding of P1, had a mean date that varied interannually by 17 days for both sexes and occurred earlier in males than in females in all years of the study. Molt duration of males regularly exceeded that of females and was greater by 2.7 days when all data were considered. Males were consistently trapped more often than females throughout the 7-week period of molt with captures being fewest for both sexes during Weeks 3-5. Duration of molt was unaffected by age in either sex but onset of molt occurred earlier in yearling males (many of which were unpaired) than in older males. Molt duration was fairly constant in both sexes regardless of molt onset but it was notably longer in the earliest group of males to begin molt and shorter in the latest group of females. Ovaries were invariably regressed with small follicles at onset of molt although brood patches ranged from being regressed to fully functional. Testis and cloacal protuberance lengths were variable at molt onset, tending to be about midrange in size but ranging from near seasonal maxima to minima. Cessation of reproduction (indexed by laying date of a pair's last egg of the season) was followed more closely by onset of molt as the season progressed. Eventually the interval between these functions decreased by about 30 days in females and 40 days in males. In birds that nested latest in the season, molt sometimes began when they still had eggs. In fact, induction of molt was found to occur at all stages of the nesting cycle, leading at times to extensive overlapping of molting and reproductive activities. At least some overlap occurred in about three-fourths of the birds because that fraction went into molt before fledglings achieved independence. Comparisons were made of molt and body mass dynamics and of reproductive success in two cohorts, one that began molting with nestlings or earlier (Group 1) and the other with fledglings or later (Group 2). Responses of males and females were dissimilar. Neither molt onset date nor its duration were different in males of the two groups although some individuals in Group 1 did show a marked delay in shedding P2 and P3. Group 1 males were heavier through molt than Group 2 males. In contrast, molt onset date was also unaffected in females but its duration was about 3 days longer in Group 1. The P2-P3 shedding delay occurred consistently in Group 1 females resulting in a decrease in the number of primaries growing simultaneously during the second week of molt. Later, after fledglings were independent, molt was accelerated in Group 1 females, not because shedding intervals decreased, but because growth rates increased in P3-P9. Unlike males, body masses of females tended to be lower in members of Group 1 until the final weeks of molt. We attribute these sexual differences to changes in energy demands caused by altered parental roles. Group 1 males provided less care than those in Group 2. Thus, temporal overlap of molt with reproductive functions was minimized in Group 1 males and maximized in their mates. This shift to uniparental care resulted in a decrease in the proportion of nests fledging young and in the number of chicks fledged per nest by Group 1 birds. We suggest that this situation exists because migratory birds summering at high altitude try to breed for as long as possible but they must also complete their molt and become able fliers before the advent of winter conditions. The interactions of these opposing forces has led to selection for a control mechanism that schedules molt independently from the termination of reproduction.