Moult phenology advances under hot or dry conditions for two passerines across 16 years in a Mediterranean climate

Climate change is driving phenological shifts in migration and reproduction, yet it remains unclear how moult, the stage through which birds replace degraded feathers, is affected. Moult is a constitutive element of self-maintenance and survival and therefore investigating shifts in moult is pivotal for advancing our understanding of avian responses to climate change. Drawing on life-history theory, we proposed four non-mutually exclusive hypotheses to explain pre-basic moult phenology in a Mediterranean climate with a prolonged drought period. Specifically, we hypothesized that birds advanced their primary feather moult in response to (1) increased temperature, (2) decreased precipitation, with (3) the strongest effects of temperature in the driest years, and (4) an inverse relationship between moult start date and duration. We also investigated whether the median brood patch date (as a metric of breeding phenology) was a strong predictor of moult start date. We leverage 16 years of data for two passerine species and used Zucchini–Underhill models with multiple regressions to elucidate these patterns. Bushtits Psaltriparus minimus had a 27-day advancement in moult start date in the hottest years compared with the coolest, and had an inverse relationship between moult start date and duration. Song Sparrows Melospiza melodia had an 18-day advancement in the driest years compared with the wettest. We did not find any interaction effects between temperature and precipitation on moult. Finally, median brood patch date was not a significant predictor of the annual variability in primary moult start date for either species. The observed plasticity in moult phenology suggests long-term advancement of pre-basic moult timing for Bushtits and increased stochasticity for Song Sparrows as climate change intensifies. Our results demonstrate that moult phenology, similar to migration and reproduction, responds to changes in environmental conditions.