Abstract
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.