Clutch-Size Determination in Precocial Birds: A Study of the Common Eider

Abstract

-Clutch-size determination in the Common Eider (Somateria mollissima) was evaluated in a colony in northern Norway. Females in good body condition (determined from mass at hatching) produced large clutches and had a shorter incubation period than females with small body reserves. Females in good body condition cared for young (including adoption of the young of other females), while females in poor body condition abandoned their young soon after hatching. Repeatability (an upper limit to heritability) of clutch size, which for individual females varies from three to six eggs, does not differ significantly from zero. A hypothesis is proposed, suggesting that there is a trade-off involved in allocating body reserves to eggs, incubation, and care of chicks and that females use a particular clutch-size strategy related to their body condition and ability to care for young. Received 8 May 1992, accepted 12 May 1993. LACK (1947, 1948) SUGGESTED that clutch size in altrical birds was ultimately limited by the ability of the parents to rear young. Although Lack's hypothesis has been somewhat modified (see Williams 1966, Charnov and Krebs 1974, H0gstedt 1980), the idea of brood-size limitation has been supported by many studies (for reviews, see Klomp 1970, Dijkstra et al. 1990). The adaptive significance of clutch size in birds with self-feeding precocial young is an enigma (Arnold and Rohwer 1991). Clearly, Lack's hypothesis for clutch-size determination in altricial birds cannot apply to precocial birds. A number of other hypotheses have been proposed (see Winkler and Walters 1983). The one most often applied is the egg-production hypothesis (Winkler and Walters 1983, Arnold and Rohwer 1991). This hypothesis was proposed by Lack (1967, 1968), who suggested that clutch size ultimately is limited by the hen's ability to allocate nutrient reserves to egg laying. However, except for studies on arctic-nesting geese (Ankney and MacInnes 1978), evidence corroborating the hypothesis is scarce. A number of studies have shown that female waterfowl use body reserves for egg production (Ankney et al. 1991). However, use of body reserves for egg production has uncritically been used as evidence that available body reserves determine the optimal clutch size (Arnold and Rohwer 1991). Even if nutrient reserves influence number of eggs produced, there may be a trade-off between the use of body reserves for egg production and for later use during incubation (Erikstad 1986, Gloutney and Clark 1991) and care of chicks (Lessells 1986, Bustnes and Erikstad 1991). In this study we examine some of the hypotheses that have been put forth as possible explanations for within-season variation in clutch size. We outline a new hypothesis that combines several earlier ideas to show that clutch-size determination could be affected by an interaction of female body condition, egg predation, and parental care. MATERIALS AND METHODS The study was carried out in a Common Eider (Somateria mollissima) breeding colony near Troms0 in northern Norway (69?49'N, 18?15'E) in 1986-1989. The colony was on a 0.65-km2 island (Grind0ya) and contained 400 breeding pairs. Searches were begun in early May when first nests were initiated. Nests were visited at oneor two-day intervals to determine laying dates and clutch sizes. A clutch was assumed to be complete and incubation to have begun when no new eggs were recorded during a period of three days. We determined intraspecific nest parasitism by detecting multiple eggs laid in a nest within 24 h. The frequency was very low (less than 1%; unpubl. data). All parasitic clutches were excluded from the analysis. Incubation was assumed to begin on the day that the final egg was laid and was completed when one chick had hatched. Females were caught on the nest with a net at hatching and weighed using a spring balance (? 10 g). For a few females caught three to five days before hatching, we estimated hatching mass by using a daily mass