Mating system and mate choice of Tengmalm's Owls Aegolius funereus

The mating system and mate choice of Tengmalm's Owls were studied in relation to population fluctuations of the staple food (voles) for seven years in western Finland. Three age classes of owls were differentiated: first‐year, second‐year and older owls. Despite a surplus of males, 11 % of them showed simultaneous bigyny in peak vole years. The mean distance between primary and secondary nests was 1158 m, and usually there was a free nest‐box between the two nests. Because primary nests produced more fledglings than secondary ones and secondary nests produced fewer fledglings than simultaneous or even eight‐days‐later monogamous nests, this bigyny could be best explained by the ‘deception’ hypothesis. In successive biandry, females deserted their first brood when young were still in the nest, thereby transferring the burden of raising the young to their males. The tendency of assortative mating by age (more yearling x yearling and + 2‐year‐old ×+ 2‐year‐old pairs than expected by chance) showed that females chose older males when these were available.