Intraspecific variation in egg composition and yolk formation in the common guillemot (<i>Uria aalge</i>)

Abstract

Common guillemot eggs were collected and analysed in two years on Skomer Island, Wales, to investigate the effects of environmental conditons during the pre‐laying period on egg formation and composition. Pre‐laying attendance at breeding colonies indicated more favourable environmental conditions in 1987 than in 1986. The size of eggs did not differ between years, but eggs were laid earlier in 1987 and the time taken for yolk deposition was also shorter in 1987. Two distinct effects of environmental conditons on yolk formation were indicated: first, on the timing of initiation of yolk deposition; and secondly, on the duration of the yolk deposition period. Egg composition also differed significantly between years with a higher proportion of albumen and a higher proportion of lipid and non‐lipid solids in yolks in 1987. These results are discussed with respect to environmental conditions and the constraints on egg production.