Large brains in prey may allow adoption of anti-predator behavior that facilitates escape. Prey species with relatively large brains have been shown to be less likely to fall prey to predators. This leads to the hypothesis that individuals that have been captured by predators on average should have smaller brains than sympatric individuals. We exploited the fact that Eurasian pygmy owls  Glaucidium passerinum hoard small mammals and birds in cavities and nest-boxes for over-winter survival, allowing for comparison of the phenotype of prey with that of live conspecifics. In Northern Europe, main prey of pygmy owls are voles of the genera  Myodes and  Microtus , while forest birds and shrews are the most important alternative prey. Large fluctuations (amplitude 100-200-fold) in vole populations induce rapid numerical responses of pygmy owls in response to main prey populations, which in turn results in varying predation pressure on small birds. We found, weighed and measured 153 birds in food-stores of pygmy owls and mist-netted, weighed and measured 333 live birds of 12 species in central-western Finland during two autumns with low (2017) and high (2018) pygmy owl risk. In two autumns, individuals with large brains survived longer compared to individuals with small brains. Avian prey of pygmy owls had smaller heads than live birds in autumn 2018 when predation risk by pygmy owl was high, while a similar difference was not significant in 2017 when predation risk by pygmy owls was reduced.  Finally, avian survivors were in better body condition than avian prey individuals. These findings are consistent with the hypothesis that pygmy owls differentially prey on small birds that are in poor body condition and have small brains, and that predation risk imposed by pygmy owls on small birds in boreal forests varies depending on the abundance of the main prey (voles).

Fieldwork. Analysed with JMP (https://www.jmp.com/)