Fig. 2. Estimated median proportion (and 95% confidence intervals) of juvenile little owls surviving from fledging in May until the first breeding season in the following year depending on the harshness of winter (left panel: harsh winter with 60 days of snow cover, right panel: mild winter with no snow cover), and whether they received supplementary feeding as nestlings (blue: unfed natural nestlings; red: nestlings provided with supplementary food).
Besides the effect of snow cover, we also found evidence that survival increased with body mass (β = 0.480; 0.290 ̵̶ 0.670), and with supplementary food provided to nestlings (β = 0.345; -0.073 ̵̶ 0.758), Survival was only marginally higher for males than for females (β = 0.153; -0.256 ̵̶ 0.521; Fig. S1). We found that the effects of body mass (Fig. S2) and food supplementation (Fig. S3) on survival only occurred during the post-fledging period, but not later in the first year of juvenile little owls. However, those differences resulted in a 23.5% lower proportion of juveniles surviving the first year depending on whether nestlings had received supplementary food (annual survival 0.233; 0.127 – 0.373) or not, solely due to the difference in post-fledging survival (Fig. 2). Out of all the juveniles that died during the first year, the greatest proportion (39.7% – 59.9%) perished during the post-fledging period, while even harsh winters accounted for only 11.1% - 27.3% of annual mortality (Table 2). Extrapolated seasonal survival estimates for birds of specific mass, sex, and feeding status are provided in Table S1.
Table 1. Estimated survival probabilities of juvenile little owls of average body mass in southwestern Germany during the four seasons of their first year, and cumulative annual survival. Mild winter refers to winters without snow cover, harsh winter refers to 60 winter days experiencing snow cover of ≥ 1 cm.