The Adaptive Significance of Egg Size and Laying Date in Thick‐Billed Murres Uria Lomvia

Abstract

The effect of egg size and laying date on the fledging mass of Thick—Billed Murre chicks was examined at Cape Hay, Bylot Island, and Coburg Island, Northwest Territories, Canada, to test the idea that late—laying females face a trade—off between the time of egg laying and egg size. There was a seasonal decline in egg volume, and egg volume was correlated with mass and size (wing length) of chicks soon after hatching. Partial correlations showed that larger eggs hatch into heavier, but not larger chicks. Chics hatched early in the season fledged at heaver masses than late—hatched chicks. Hatching mass contributed significantly to this effect, but other factors (e.g., energy intake) were probably also important. A model, based on a multiple regression of the effects of hatching mass and hatching date on fledging mass, shows the possible combinations of egg size and timing of breeding while maintaining a constant fledging mass. Data on the replacement interval and size of replacement eggs suggest that females that lay late may do better (in terms of fledging mass of their chick) to minimize the delay in laying by producing a small egg earlier, rather than delay laying further in order to produce a larger egg. This is because they are unable to accumulate nutrients at a fast enough rate to make a delay worthwhile. This suggests that the seasonal decline in egg size is an adaptive response to a seasonal deterioration in environmental conditions.