Climatic variation affects seasonal survival of an alpine bird species

Abstract

Mountains are particularly exposed to climate change, and empirical studies haveshown that montane bird species are highly sensitive to the ongoing changes. Modellingmortality risks under climatic variation will give insight into species-specific sensitivity.Willow ptarmigan Lagopus l. lagopus is a common resident species in many northernalpine ecosystems, with an important role in predator–prey dynamics. Willow ptar-migan moult into white feathers during autumn, which provides camouflage in thewinter snow. With shorter periods of snow cover, a phenological mismatch betweensnow cover and moulting time may affect mortality risk. Simultaneously, snow covermay affect mortality risk through limiting the access to nutrient-rich food and increas-ing risk-taking. Detailed relationships between snow cover and mortality risk requirehigh-resolution data to uncover. We collected data from willow ptarmigan (n = 272)equipped with VHF transmitters over seven years in central Norway. Field data collec-tion was performed on a monthly basis, except in the darkest period of winter. UsingCox proportional hazards models, we estimated whether variation in mortality risk wasrelated to variation in snow cover in the spring and autumn seasons. We found thatan 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 snowin the spring. In autumn, mortality risk differed only between sexes. In our study wehave shown that spring survival in a common montane species is affected by climaticvariation, where the magnitude of effects on population dynamics will depend on theage distribution in the population. We hypothesise that there are competing risks con-nected with a moulting mismatch versus increased risk-taking under limited access tofood resources, that may have different importance under different conditions. age, alpine ecosystem, autumn, climate, colour change, competing risks,grouse, individual-based, lagopus, mortality, snow, spring, willow ptarmigan