Winter survival of a small predator is determined by the amount of food in hoards.

The hoarding behaviour of animals has evolved to reduce starvation risk when food resources are scarce, but effects of food limitation on survival of hoarding animals is poorly understood. Eurasian pygmy owls (Glaucidium passerinum) hoard small mammals and birds in natural cavities and nest boxes in late autumn for later use in the following winter. We studied the relative influence of the food biomass in hoards of pygmy owls on their over-winter and over-summer apparent survival. We also tested whether this influence is modulated by intrinsic (age, sex) traits or extrinsic factors (winter temperature, snow depth). We measured biomass of prey items in pygmy owl food-hoards during autumns 2003-2023 in west-central Finland. We individually marked and recaptured pygmy owls both at nests in the breeding season and at food-hoards. Our dataset included a total of 407 pygmy owls, which were all captured from a food-hoard at least once during their capture history. The mean biomass of the annual food-hoards associated with one individual was 443 g (SD = 523 g, range from 3.5 to 4505 g) and was markedly higher in autumns of vole abundance than in those of vole scarcity. Hoard size had a positive effect on apparent survival of owls over consecutive winter, whereas it did not affect apparent survival over next summer. Hoard size was a better predictor of apparent survival than vole abundance (main food of pygmy owls) in the field. Male owls had higher overall apparent survival rates than female owls, particularly when food-hoards were small. That hoard size was a better predictor of apparent survival than vole abundance indicates that the hoards are critical for pygmy owls during winter, likely because they are unable to hunt voles below deep snow cover. The positive relationship between apparent survival of owl individuals and their hoard size during winter (when the hoard is being consumed), but not summer, indicates that the hoard size has a true positive effect on survival, and does not only reflect latent inter-individual differences and/or dissimilarities in their environments. We conclude that food limitation during hoarding essentially regulates apparent over-winter survival of pygmy owl individuals.