Beyond Circannual Fattening: Behavioural Flexibility and Sex-Specific Strategies Enable Coping With High-Elevation Winters

Winter survival at high elevations imposes severe physiological and behavioural constraints on resident birds, necessitating specialised adaptations to cope with low temperatures, high energy demands, and limited food availability. We studied physiological and behavioural strategies of the White-winged Snowfinch (Montifringilla nivalis nivalis), a small passerine that inhabits high mountains year-round. Using GPS tracking and morphological measurements, we examined seasonal changes in body mass, fat reserves, muscle mass, and lean body mass, alongside movement behaviour. Our results showed that snowfinches maintain larger fat reserves than overwintering lowland passerines, with fat accumulation regulated mainly by a circannual programme, likely reflecting a physiological adaptation to the high-elevation environment. Seasonal changes in behaviour, including sex-specific strategies and flexible movement patterns, appear to play an additional key role in coping with winter challenges in high mountains. Analysis of how temperature affects body mass indicated that fat accumulation is determined mainly by long-term rather than short-term fluctuations. These findings suggest that ongoing climate change, characterised by rising mean temperatures and more frequent extreme events, could disrupt this preparatory strategy, leading to reduced fat reserves and more frequent exposure to adverse winter and spring conditions.