Fall advances in the timing of molt in birds in the southwestern United States

Molt is a critical event in the annual cycle of birds. Although we know an increasing amount about the impacts of climate change on the timing of other avian events, there has been relatively limited work conducted on changes in molt phenology over time. In this study, we utilized a thirteen-year bird banding dataset from southeastern Utah to examine long-term trends in the molt timing of body and flight feathers during both the spring and fall migratory seasons, accounting for temporal trends in non-molting birds and how trends may vary between different sexes and ages of birds. We found that there were no significant temporal trends in molt timing in the spring but there were significant trends in the fall, such that birds were advancing the timing of their body and flight feather molt over time. Finally, we highlight the significant influence of climate on molt phenology: El Niño/Southern Oscillation and maximum temperature were both associated with advances in spring body molt, maximum temperature was associated with delays in fall flight feather molt timing, and precipitation was associated with advances in both fall body and flight feather molt timing. This study provides the first examination of long-term trends in the molt phenology of North American birds, showing that, over the past decade, birds in the western United States are advancing their feather molt timing in the fall at a rate of roughly 1 day/year.