The cost of mate guarding in the Common Eider

Abstract

A male biased sex ratio in Finnish Common Eiders (Somateria mollissima) results in un-mated males interacting with eider pairs. We quantified the effects of mate guarding on other behaviors. Mated males spent less time feeding and performed fewer dives than un-mated males in 2003, but not in 2004. A cold winter in 2003 implies that eiders arrived on the breeding grounds in poorer condition making feeding more critical. Mated males were dominant over unmated ones, winning two thirds of their aggressions. Mated males had fewer aggressive encounters but spent a greater proportion of their time in aggression. Mated males also seemed to spend extensive energy guarding their mate during infre-quent but intensive "harassments", lasting up to 165 minutes, in which 6 to 23 males chased one female. Harassments became more frequent later in the season, when females started incubating and the male bias was most pronounced. These harassments, and the fact that unmated males often approached other ducks, suggest that unmated males are trying to access females. Given the current trend of increasing male bias, both the cost of being mated and the fitness benefits of an alternative mating strategy may increase.