An Explanation of Egg-Weight Variation in the Lesser Snow Goose

Abstract

Lesser snow goose (Chen caerulescens) eggs were weighed at the McConnell River, N.W.T., in 1972 and 1973. Mean egg weight was the same in different size clutches. Within each clutch size there was great variability in egg weights; the lightest eggs weighed only 60 percent as much as the heaviest. This variation was greater than that reported for other waterfowl species. The egg-weight variation resulted in overlap between the weight of the heaviest clutches of a particular size and the lightest clutches of the next larger size. Hence, the nutrient commitment to a particular clutch size was not the same for all females. An hypothesis is presented to explain the constancy of mean egg weight among clutch sizes and the great variation in egg weights within each clutch size. We assume that there is a heritable component in egg weight of the lesser snow goose and that heavier eggs enhance gosling survival under energetically poor conditions. Thus, we argue that the mean egg weight is adapted to the average environmental conditions at hatch and propose that a wide range of egg-weight genotypes survives because of the great annual variation in environmental conditions at hatch. J. WILDL. MANAGE. 40(4):729-734 An important factor in the evolution of clutch size in birds that do not feed their young is the size of egg that provides adequate nutrient reserves for the newly hatched young (Lack 1967, 1968a:233-234, Ryder 1970, Klomp 1970). This optimum egg size generally is presumed to be relatively constant in each species of the Anatidae (Lack 1967). During a study of the reproductive biology of the lesser snow goose .(including white and blue color phases), data on clutch size and egg weight were obtained (Ankney 1974:97). Here, using egg weight as an index of the amount of nutrients allocated to individual eggs, we evaluate the possible significance of eggweight variation in the lesser snow goose. We are grateful to C. D. MacInnes, whose help and ideas were the impetus for the study from which these data resulted. L. Patterson provided valuable field assistance. We thank D. M. Scott, J. S. Millar, H. Boyd, and C. D. MacInnes for critically reviewing earlier drafts-of this paper. Financial support was provided by the Canadian Wildlife Service, National Research Council of Canada, and The University of Western Ontario. MATERIALS AND METHODS We weighed 1,457 eggs from 366 clutches from the snow goose nesting colony at the mouth of the McConnell River, Northwest Territories (60'50'N, 94025'W), in 1972 (C.D.A.) and 1973 (A.R.B.). In both years we used a spring scale (Welch Scientific Co., Chicago) to weigh individual eggs at the nest. The scale had a 250-g capacity calibrated in 2-g intervals; we used a known weight to check its accuracy periodically. Weights were recorded to the nearest gram. Clutch weight was determined by summing egg weights. In both years we weighed only eggs from nests (on a transect) that had a known history. In 1972, eggs were weighed when the clutch was complete; in 1973, eggs were weighed the day, or the day after, they were laid. This between-year difference in technique may have increased the variability of the data. Eggs lose weight after being laid, although nearly all weight loss occurs after incubation begins (Romanoff and Romanoff 1949:378) and the lesser snow goose does not begin incubation until the clutch is complete (Cooch 1958:42). Also, mean egg weight was not different in the J. Wildl. Manage. 40 (4):1976 729 This content downloaded from 157.55.39.127 on Tue, 28 Jun 2016 07:17:55 UTC All use subject to http://about.jstor.org/terms 730 EGG WEIGHTS IN LESSER SNOW GEESE Ankney and Bisset Table 1. The mean egg weights and clutch weights (g) within each clutch size and the percentage overlap between clutch weights of different sizes in the lesser snow goose at the McConnell River, N.W.T., 1971 and 1972.