Poor reproductive success of polygynously mated female Tengmalm's owls: are better options available?

Abstract

Mating options available to female Tengmalm's owls, Aegolius funereus, that paired with already-mated males were studied for four good vole years in western Finland. Thirty-four cases of bigyny and three of trigyny were recorded. Clutch sizes of primary and secondary/tertiary females did not differ, but the latter produced significantly fewer and lighter fledglings than the primary or monogamous females laying at the same time. The mean± sd distance between the nests of secondary/tertiary females and the closest unmated male (i.e. the closest available mate) was 2462±1288 m. Seventy-six per cent of these unmated males were so near (<3 km) that their territorial calls should have been audible to secondary/tertiary females. These unmated males occupied slightly poorer territories than mated males, but 97% of them had territories that provided for a breeding pair in good vole years. By pairing with them, instead of a mated male, secondary/tertiary females would have raised more and heavier fledglings. The poor reproductive success of polygynously mated females was because, though males fed their secondary/tertiary females during the courtship feeding, egg-laying and incubation periods at the expense of incubating primary females, primary families were favoured later on. This, polyterritoriality and a lack of polygny in poor vole years suggest that already-paired males on high-quality territories may ‘imitate’ the food-provisioning rates of unmated males in good vole years, thereby ‘deceiving’ females into polygyny.