The Role of Female Age in Determining Egg Size and Laying Date of Thick-Billed Murres

Abstract

In many bird species, egg and/or clutch size increases with female age, and declines with laying date. The declines with date have been considered time-saving adaptations to seasonal environments. At Coats Island, N.W.T., Canada in 1994 and 1995, we examined the relationships among female age, laying date and egg size in the Thick-billed Murre Uria lomvia, an Arctic seabird that lays a one-egg clutch. In particular, we tested a model that views the seasonal decline in egg size in this species as a time-saving adaptation: the model predicts that, irrespective of their age, late-laying birds will reduce egg size. Laying dates became earlier with increasing female age to 8 years, because young birds initiated egg formation late. Egg size increased with age to 8 years, possibly because young birds deposited yolk at a slow rate. Individual effects explained much of the variation in laying date and egg size. Egg size declined with laying date in a random sample of eggs laid by birds of unknown age. However, egg size was independent of laying date among older birds. In contrast, egg size declined with laying date among young birds, but this was an effect of age, not date. Young birds that laid at the same time as older birds laid smaller eggs. These results indicate that the time-saving model does not adequately explain the seasonal decline in egg size in Thick-billed Murres. We suggest that characteristics of individual Thick-billed Murres, largely mediated by age (or experience) during early reproductive attempts, determine the size of egg they lay.