The influence of diet quality on clutch size and laying pattern in mallards

-We measured the effect of diet quality on variation in the seasonal pattern of Mallard (Anas platyrhynchos) reproduction. Twenty wild-strain hens, consisting of 10 sibling pairs, were maintained in captivity. One sib of each pair was fed an enriched diet, and the other was fed wheat. The wheat diet resulted in reduced clutch size, egg size, laying rate, number of nesting attempts, and total eggs laid. Diet did not affect laying initiation, duration, or the seasonal pattern of change in clutch and egg size with each renest. We believe the variation and pattern observed are adaptations to a highly variable prairie environment where the probability of reproductive success decreases as the season progresses. Received 20 April 1987, accepted 4 October 1987. WILLIAMS (1966) observed that the central biological problem is not survival as such, but design for survival. Inherent in this design is a set of coadapted reproductive traits evolved through natural selection in response to particular ecological problems (Stearns 1976). The traits include variables such as age of first reproduction, clutch size, size of young, pattern of seasonal reproductive investment, and level of reproductive effort. Variables incorporated in a life-history strategy are commonly assumed to be subject to normalizing selection, and the mean value is considered more indicative of an adaptation than the variance. For example, when a consistent clutch-size pattern is observed, as when the average clutch size is smaller than the most productive, an ultimate, adaptive explanation seems appropriate. But when clutch size varies with environmental changes such as food availability or female condition, proximate explanations are often provided (cf. Klomp 1970, Johnsgard 1973, Winkler and Walters 1983). There is no a priori reason to assume a facultative response to environmental change is proximate or that an obligate response is adaptive (Stearns 1976). For example, female carabid beetles have longer reproductive lives when they facultatively re' Present address: Department of Ecology and Behavioral Biology, Bell Museum of Natural History, University of Minnesota, 10 Church Street, Minneapolis, Minnesota 55455 USA. duce reproductive investment during poor conditions (Murdoch 1966). Phenotypic plasticity may be important in the evolution of life-history strategies. Ultimate and proximate causation are not necessarily mutually exclusive, and, indeed, food supply may play a dual role (Winkler and Walters 1983, Winkler 1985). Our understanding of the effect of food supply on life-history variation is imperfect. For this reason it is useful to determine in a single species which reproductive traits are flexible and which are fixed in the face of variation in food supply. We considered the effect of diet quality on variation in the seasonal pattern of Mallard (Anas platyrhynchos) reproduction. We specifically tested for variation in the timing of laying initiation and duration, total eggs produced, egg size and composition, rate of egg laying, clutch size, interval between clutches, and seasonal clutch-size decline.