Allocation of Limited Reserves to a Clutch: A Model Explaining the Lack of a Relationship between Clutch Size and Egg Size

Abstract

Lack (1967, 1968) proposed that clutch size in waterfowl is limited by the nutrients available to females when producing eggs. He suggested that if nutrients available for clutch formation are limited, then species producing small eggs would, on average, lay more eggs than species with large eggs. Rohwer (1988) argued that this model also should apply within species. Thus, the nutrient-limitation hypothesis predicts a tradeoff among females between clutch size and egg size (Rohwer 1988). Field studies of single species consistently have failed to detect a negative relationship between clutch size and egg size (Rohwer 1988, Lessells et al. 1989, Rohwer and Eisenhauer 1989, Flint and Sedinger 1992, Flint and Grand 1996). The absence of such a relationship within species has been regarded as evidence against the hypothesis that nutrient availability limits clutch size (Rohwer 1988,1991, 1992; Rohwer and Eisenhauer 1989). Failure to detect a negative correlation between clutch size and egg size is not necessarily evidence against regulation of clutch size by nutrient reserves. If both clutch size and egg size are correlated with a third variable, then variation in the third variable could conceal a correlation between clutch size and egg size at the population level. In this paper we discuss evidence that both nutrient reserves and egg size are positively correlated with body size for species that rely on reserves for egg production, and we explore how these correlations might influence detection of tradeoffs between clutch size and egg size. We have assumed that individual females have a fixed amount of reserves available for the entire reproductive attempt (i.e. clutch formation and incubation) and a phenotypically fixed egg size. Waterfowl, in general, show high repeatability of egg size, suggesting that females cannot (or do not) alter their egg size in response to nutrient reserves or environmental conditions (e.g. Flint and Grand 1996). Reserves are used for egg production, but optimal investment in the clutch is less than the maximum possible (Fig. 1) because reserves are retained for use during incubation (Klomp 1970, Ryder 1970, Raveling 1979, Ankney 1984, Ankney and Alisauskas 1991, Gloutney and Clark 1991, Afton and Paulus 1992, Er-