Female pre-nesting foraging and male vigilance in Common Eider Somateria mollissima

Abstract

Foraging activity and behaviour of paired female Common Eider from arrival to laying at Saltholm, Denmark, was studied during the spring of 1994 and 1995 to assess the importance of female resource accumulation just prior to egg-laying. Ovary development was studied in paired females collected in 1995, 1996 and 1998. Male activity and behaviour were recorded in 1996 in order to assess the importance of mate-guarding in optimizing female resource acquisition. From arrival at the colony in late March until laying in mid-April, female Eiders allocated on average 62% of the daylight hours to foraging by dabbling in inshore waters (<1 m water depth). In offshore waters (1–4 m depth) foraging by diving averaged 74% of the daylight hours. Paired males foraged an average of 17% of the day time in the inshore habitat, and closely attended their females during this time. The males actively defended their females in encounters with other pairs and from single males. Defence events involving encounters with single males increased significantly from 9.2 times per hour to 17.5 times per hour by the time of laying. The frequency of interruption to female foraging did not change significantly as a result of the more frequent encounters towards the time of laying, being 4.0 times per hour before early April and 5.8 times per hour at the time of laying. Female ovary weights showed a significant increase from early April, indicating that resources obtained locally were allocated to egg production. The increase in encounters with single males suggests that extra-pair copulation may be a secondary reproductive strategy in male Eiders at the time their females start to incubate. The present results emphasize the importance of pre-nesting foraging for female Eiders in maximizing the nutrient intake in preparation for reproduction, and the importance of the behaviour of attendant males in protecting female foraging from interruptions from other males.