R codes and data for: Early snowmelt negatively affects nestling growth and first-year survival in an alpine bird

Abstract Many birds have advanced their breeding phenology in response to rising temperatures, but these adjustments may be insufficient to fully track environmental conditions. Insufficient phenological adjustments, in turn, can lead to unfavourable conditions during reproduction with potential negative fitness consequences for the offspring. We used nestling ringing data of white-winged snowfinches Montifringilla nivalis, a high-elevation bird species, to assess how variation in temperature and snow conditions affected nestling growth and first-year survival. Our results suggest that breeding after snowmelt impairs nestling growth and leads to lower body mass, shorter tarsi and shorter wings. In years when snowfinches were breeding late relative to the timing of snowmelt, first-year apparent survival dropped from around 50% to roughly 20%. The reduced apparent survival in years with an early snowmelt might at least partly explain a recent population decline of the species. As snowmelt is expected to advance even further in the future, we expect reproductive success to decrease. These findings improve our understanding of the underlying mechanisms of how rising temperatures and changing snowmelt patterns are linked to fitness and ultimately population dynamics.