Fig. 1. Estimated median survival probabilities of juvenile little owls of average body mass during the four seasons across their first year of life. Survival probability is scaled to biweekly encounter occasions, and therefore comparable across seasons despite the different duration of each season. Error bars represent 95% credible intervals.
Biweekly survival decreased strongly with an increasing duration of snow cover (β = -0.389; 95% credible interval -0.571 ̵̶ -0.201; Fig. 1, Fig. S1). By contrast, we found only a weak negative effect of snow cover on detection probability (β = -0.229; -0.468 ̵̶ 0.022; Fig. S1), and detection probability was very high (in periods with normal effort: 0.994; 0.989 ̵̶ 0.996; in periods with reduced effort: 0.803; 0.625 ̵̶ 0.892). In winters with extreme snow cover, biweekly survival decreased from 0.971 to 0.879 (Fig. 1), and the survival over an entire harsh winter season was therefore even lower than during the post-fledging summer (0.481; Table 1). In total, the annual survival probability of little owls during their first year of life ranged from 0.117 to 0.178 depending on the severity of the winter (Table 1). Thus, in years with long periods of snow cover, the winter period reduced first-year survival by 34.3% compared to a snow-free winter (Fig. 2).