Juvenile survival of little owls decreases with snow cover

Global environmental changes are predicted to lead to warmer average temperatures and more extreme weather events thereby affecting wildlife population dynamics by altering demographic processes. Extreme weather events can reduce food resources and mortality, but the contribution of such events to demographic processes are poorly understood. Estimates of season-specific survival probabilities are crucial for understanding mechanisms underlying annual mortality. However, only few studies have investigated survival at sufficient temporal resolution to assess the contribution of extreme weather events. Here, we analysed biweekly survival probabilities of 307 radio-tracked juvenile little owls (Athene noctua) from fledging to their first breeding attempt in the following spring. Biweekly survival probabilities were lowest during the first weeks after fledging in summer and increased over autumn to winter. The duration of snow cover in winter had a strong negative effect on survival probability, while being well fed during the nestling stage increased survival during the first weeks after fledging and ultimately led to a larger proportion of birds surviving the first year. Overall annual survival probability over the first year varied by 34.3 % between 0.117 (95 % credible interval 0.052 – 0.223) and 0.178 (0.097 – 0.293) depending on the severity of the winter, and up to 0.233 (0.127 – 0.373) for well-fed fledglings. The season with the lowest survival was the post-fledging period (0.508; 0.428 – 0.594) in years with mild winters, and the winter in years with extensive snow cover (0.481; 0.337 – 0.626). We therefore show that extreme weather events reduced the proportion of first-year survivors. Increasingly warmer winters with less snow cover may therefore increase annual survival probability of juvenile little owls in central Europe, but environmental changes reducing food supply during the nestling period can have similarly large effects on annual juvenile survival and therefore the viability of populations.