DOES THE COST OF INCUBATION SET LIMITS TO CLUTCH SIZE IN COMMON EIDERS SOMATERIA-MOLLISSIMA

We examined the effect of natural clutch size on the cost of incubatio n in a population of common eiders Somateria mollissima nesting in Tro mso, northern Norway. The body condition of females at day 5 in the in cubation period was not related to clutch size (3-6 eggs), but females incubating large clutches lost more mass and had a lower body conditi on at day 20 in the incubation period than females incubating small cl utches. Females incubating large clutches had a slightly shorter incub ation period and a lower egg predation rate. The results do not suppor t the hypothesis that the female's ability to produce eggs is the only ultimate control of clutch size in elder Instead the results suggest that there may be an interaction between the allocation of body reserv es to eggs and incubation, and that females producing large clutches a llocate more of their body reserves to incubation than females produci ng small clutches, in order to shorten the incubation period and to mi nimise the risk of predation on eggs.