COS 299-2 - CANCELLED - Annual climate conditions and individual age impact the timing of the adult pre-basic molt among four temperate passerine species

Abstract: Climate change is driving phenological shifts in the reproductive and migratory stages of the avian life cycle, yet it remains unclear if the timing of molt, when birds replace degraded feathers, is also changing. Molt is an energy and resource intensive period that is critical for self-maintenance, therefore modifications to this stage may have survival and fitness consequences. To better understand how climatic conditions may influence molt phenology, we examined over three decades of molt data for adults of 11 near-passerines and passerine species found within a Mediterranean climate in Northern California. The molting periods of our study species straddle the transition from the productive period of vegetation growth to an annual summer drought, where resources become limited. Based on life history theory, we hypothesized (1) that our study species advanced the timing of their adult pre-basic molt during hot and dry years, and (2) that younger adult birds (SY/SPB) molted earlier than older birds (ASY/MTPB) under these same conditions. We fit mixed effects models in a Bayesian framework to assess the effects of precipitation and temperature metrics, along with individual age, on the Julian day a bird was captured molting flight feathers. Consistent with our hypotheses, Bushtits and Song Sparrows advanced their molt in warmer years, and – along with Common Yellowthroats – advanced their molt in wetter years. Younger adult Chestnut-backed Chickadees molted earlier than older adults. These findings provide evidence some species are shifting molt timing to optimize resource availability under variable climatic conditions, and that molt and current reproductive effort are in dynamic tension. We hypothesize our study species are prioritizing molt under less favorable conditions. Interestingly, the effects of climate on molt phenology appears modulated by diet: among our 11 study species, which included insectivores and granivores, only insectivores had non-zero climate effects. Though the exact mechanisms are unknown, our findings show that birds may have plasticity in molt timing in response to climate variation and long-term phenological shifts may occur as climate change continues to accelerate.