Date of this Version
In a stimulating analysis, Lack (1967, 1968a) reviewed waterfowl clutch size and egg size data, and concluded that interspecific variations in average clutch size are generally inversely related to those of relative egg size. Thus, he suggested that the average clutch size of each waterfowl species has evolved in relation to the average availability of food to the female around the time of nesting. He hypothesized that in waterfowl, relatively large eggs have probably evolved to provide the newly-hatched young with a large food supply (an idea he subsequently (1968b) questioned) or with an adequate insulating layer of fat. He also suggested that, since annual, seasonal, and local variations in clutch size exist, proximate factors, such as the food supply of individual females, may be more likely to influence a bird's date of laying than either its egg size or clutch size. Since the publication of Lack's study, several additional reviews of avian clutch sizes have appeared, notably those of Klomp (1970), von Haartman (1971) and Cody (1971). Klomp considered the problems posed by the Anatidae in some detail, and generally agreed that the food supplies available to the female are probably the ultimate factor influencing clutch size in this group.
Because of the large amount of information available for waterfowl, and since Lack did not concern himself with proximate influences on clutch sizes of individuals, I have given these matters some attention and have reached somewhat different conclusions. No simple single explanation for inter- and intraspecific variations in clutch sizes seems possible at present, but a summary of the evidence relating to these phenomena nevertheless appears worthwhile.