High interannual variation in the hatching sex ratio of Tengmalm's owl broods during a vole cycle

The sex ratio at hatching in broods of Tengmalm’s owl (Aegolius funereus) in northern Sweden was investigated for 3 years characterized by different phases of the vole and owl cycle. Previous work showed the sex ratio in this species to be male-biased for 1 year with a favourable food supply, and that in feeding experiments male nestlings (but not females) suffered higher mortality when food was limited, but not otherwise. Here we present data from a complete 3-year owl cycle, showing that mean brood sex ratio varied significantly among years, being male-biased (65% males) in the first year of high owl breeding density, unbiased (49%) in the second year of high owl breeding density, and female-biased (33% males) in the owls’ low year. Brood sex ratio did not vary significantly within years with laying date or parental age. Vole availability, and therefore the owls’ food supply, declined during the 3 years studied. Tengmalm’s owl parents thus appear to adaptively adjust the sex ratio of their broods according to the expected annual mortality risk of sons.