Ecological Studies of the Black Swan I. The Egg, Clutch and Incubation

Abstract

Statistics are given for egg dimensions, clutch size, laying rate and incubation period in black swans nesting at Lake George and Lake Bathurst, N.S.W., and compared with data from other studies. Methods of determining clutch size and incubation dates are given and examined. Emphasis is placed on the importance of the method of assessment of clutch size in comparisons within and between studies. There was evidence of two size classes of eggs, possibly related to factors of age or breeding experience, the smaller class being laid in summer breeding, during an exceptional abundance of food, but not in winter when food may have been limiting. Egg size varied significantly, according to sequence in the clutch. The adaptive significance of egg size is discussed; moisture loss may be important in the evolution of egg size. Clutch size varied significantly according to the location and date of nesting. The reasons for the variation were, possibly, both in the nutritional value of the food available and in its relative temporal and physical availability and the swans' abllity to metabolize it. Possible evolutionary determinants of average clutch size in the black swan are considered. Moisture losses from the egg may again he important. The laying rate was simdar to that reported elsewhere. The mean incubation period was 40.45 days and most clutches ranged from 39 to 43 days. Clutches laid in autumn and winter took marginally longer to hatch than clutches laid in spring and summer, possibly due to effects of ambient temperature; the difference was probably not of particular adaptive significance.