Partial migration of White-winged snowfinches is correlated with winter weather conditions

Seasonal movements are a response to variability in resource availability and result from a complex interaction between the behavioral and physiological traits of a species and its prevailing environment. A widespread bird migration strategy is partial latitudinal migration, where some proportion of the population moves from breeding to winter grounds, while the remaining individuals stay year-round on the breeding grounds. Deciphering how and why some individuals migrate while others stay is essential to understanding population and community structure and dynamics. Little is known about the drivers of partial migration strategies of high-mountain birds that are subjected to strong seasonal environmental fluctuations and count among the species most threatened by climate change. In this study, we investigated the migratory pattern of an alpine songbird, the White-winged Snowfinch (Montifringilla nivalis nivalis), through the analysis of stable hydrogen isotopes of feathers (δ2Hf), and how it relates to weather factors. First, values of δ2Hf were used to assess the probability that snowfinches wintering in the Spanish Pyrenees and Cantabrian Mountains have a breeding origin in the Alps. Second, we analysed whether winter weather conditions (ambient temperature and precipitation) in the Alps may play a role in migratory movements towards the southern wintering grounds. Overall, ca 98% and 86% of the individuals sampled in winter in the Spanish Pyrenees and Cantabrian Mountains, respectively, were likely to originate from breeding populations in the Alps. Snowfinches also had a higher propensity for large-scale movements to the South in winters where the average monthly temperature was particularly low in the Alps, typically