Abstract Previous studies have shown that duration of the post-breeding molt in temperate-zone passerines is inversely related to migration distance. However, it is unclear whether this relationship arises directly—because long-distance migration selects for more rapid molt, or indirectly—because long-distance migration precludes double brooding and allows molt to begin earlier under more favorable environmental conditions. We investigated this issue by examining the effects of migration distance, frequency of double brooding, and wing length on the timing and duration of the post-breeding molt for 33 passerine species captured during banding operations at Powdermill Avian Research Center in southwestern Pennsylvania, USA. Consistent with predictions of the indirect pathway hypothesis, we found that long-distance migration is strongly associated with single brooding, and single-brooded species initiate flight-feather molt about one month earlier than double-brooded species. However, contrary to the predictions of the indirect pathway hypothesis, frequency of double brooding has no effect on molt duration; instead, molt duration increases with increasing wing length and decreases with increasing migration distance. The strong negative relationship between migration distance and molt duration is consistent with the direct pathway hypothesis and suggests that the time constraints imposed by long-distance migration lead directly to natural selection for rapid post-breeding molt. Our results also indicate that the rapid molt of long-distance migrants is achieved by an increase in the number of remiges being replaced simultaneously (molt intensity), and apparently also by an increase in the rate at which remiges grow. Unexpectedly, our estimates of molt duration were bimodally distributed, with 25 species showing rapid molt (40–65 days) and 8 showing slow molt (82–103 days). Overall, our findings clarify the effects of migration distance, double brooding, and wing length on phenology of passerine post-breeding molt, and highlight opportunities to expand avian life-history theory to incorporate interspecific variation in molt strategies.
Mumme et al. (Thu,) studied this question.