The cost of mate guarding in the Common Eider

Amale biased sex ratio in Finnish Common Eiders (Somateria mollissima) results in unmated males interacting with eider pairs. We quantified the effects of mate guarding on other behaviors.Matedmales spent less time feeding and performed fewer dives than unmatedmales in 2003, but not in 2004.Acoldwinter in 2003 implies that eiders arrived on the breeding grounds in poorer conditionmaking feedingmore critical.Matedmaleswere 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 infrequent but intensive ‘harassments’, lasting up to 165 minutes, in which 6 to 23 males chased one female. Harassments becamemore 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.

B.B. Steele, A. Lehikoinen, M. Ost & M. Kilpi, Ornis Fennica 84:49-56. 2007