Exploitation of others' vigilance by the Common Eider Somateria mollissima

Abstract

Individuals obtain advantages from group-living through reduced probability of predation (Hamilton 1971). In addition to reducing the perimeter exposed to predators, flocking behaviour also increases vigilance, and thereby reduces the chance of surprise attacks (Krebs and Davies 1986). In the Common Eider Somateria mollissima flocking behaviour during brood-rearing is very frequent. These groups are often called creches, and such behaviour has been explained as an adaptation for minimizing predation on the ducklings (Ahln and Andersson 1970, Gorman and Milne 1972, Bedard and Munro 1976, Munro and Bedard 1977a,b, Bustnes and Erikstad 1991a). However, females, that do not care for young themselves, are often temporarily present in or near creches (Munro and Bedard 1977a, Schmutz et al. 1982, Bustnes and Erikstad 1991b). Adult Common Eider are exposed to dangers, such as harassment by kleptoparasitic gulls (Ingolfsson 1969) and, in some areas (e.g northern Norway), predation from birds of prey such as White-tailed Eagles Haliaeetus albicilla (Norderhaug 1978). Females that care for ducklings spend 40 to 45% of a feeding bout watching for potential duckling predators (Bustnes and Erikstad 1991a). It would thus seem advantageous for a female without young to stay close to brood-caring females, thereby exploiting the high degree of vigilance that will enable them to feed more effectively (Bustnes and Erikstad 1991b). In order to test this hypothesis, I recorded time budgets of females, without young, that were feeding close to or away from creches.