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 (d2Hf), and how it relates to weather factors. First, values of d2Hf 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 < -2 °C (i.e., in the 42% coldest winters). Our results suggest that snowfinches adopt a partial migratory strategy, with different patterns among and within populations that have important implications in terms of population connectivity, spatio-temporal dynamics and structuring. Considering the role of cold winter conditions in migration propensity of snowfinches, there is a risk of increased isolation of the southern populations under a scenario of global warming, insofar as the hypothetical settling of winter immigrants could no longer contribute to a demo-genetic rescue. Future research should decipher how these risks affect alpine species that are particularly exposed to climatic shifts, and how they adapt and evolve.