Climatic variation affects seasonal survival of an alpine bird species

Mountains are particularly exposed to climate change, and empirical studies have shown that montane bird species are highly sensitive to the ongoing changes. Modelling mortality risks under climatic variation will give insight into species‐specific sensitivity. Willow ptarmigan Lagopus l. lagopus is a common resident species in many northern alpine ecosystems, with an important role in predator–prey dynamics. Willow ptarmigan moult into white feathers during autumn, which provides camouflage in the winter snow. With shorter periods of snow cover, a phenological mismatch between snow cover and moulting time may affect mortality risk. Simultaneously, snow cover may affect mortality risk through limiting the access to nutrient‐rich food and increasing risk‐taking. Detailed relationships between snow cover and mortality risk require high‐resolution data to uncover. We collected data from willow ptarmigan (n = 272) equipped with VHF transmitters over seven years in central Norway. Field data collection was performed on a monthly basis, except in the darkest period of winter. Using Cox proportional hazards models, we estimated whether variation in mortality risk was related to variation in snow cover in the spring and autumn seasons. We found that an earlier arrival of winter led to increased mortality risk for yearlings, but not adults, in the following spring. Further, spring mortality was lower when there was less snow in the spring. In autumn, mortality risk differed only between sexes. In our study we have shown that spring survival in a common montane species is affected by climatic variation, where the magnitude of effects on population dynamics will depend on the age distribution in the population. We hypothesise that there are competing risks connected with a moulting mismatch versus increased risk‐taking under limited access to food resources, that may have different importance under different conditions.