Causes and consequences of egg‐size variation in the European Starling <i>Sturnus vulgaris</i>

Abstract

Variations in egg‐size (weight, or an index of volume derived from the length and breadth of eggs) were analysed for four years' data from a colony of Starlings in southern England. Variation was much greater among clutches than within clutches. There were no consistent trends for egg‐size to increase or decrease through the laying sequence, but the first‐ and last‐laid eggs tended to be smaller than the clutch mean. Loss of single nestlings (perhaps by starvation) was more frequent if the smallest, or last‐laid egg was particularly small. Effects of egg‐size on the growth of surviving chicks were slight‐most variation in development was attributable to factors such as clutch or brood size, and nestlings' relative weights fluctuated so that they continually changed ranks during the nestling period. In Starlings, egg‐size was inversely correlated with clutch size, but three open‐nesting species (Bullfinch, Song Thrush and Blackbird) showed the opposite pattern. These results are discussed in relation to their implications for understanding the adaptive aspects of egg‐size variation.