THE ROLE OF FEMALE AGE IN DETERMINING EGG SIZE AND LAYING DATE OF THICK-BILLED MURRES

In many bird species, egg and/or clutch size increases with female age , and declines with laying date, The declines with date have been cons idered time-saving adaptations to seasonal environments. Al Coats Isla nd, N.W.T., Canada in 1994 and 1995, we examined the relationships amo ng 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, irrespec tive of their age, late-laying birds will reduce egg size. Laying date s became earlier with increasing female age to 8 years, because young birds initiated egg formation late. Egg size increased with age to 8 y ears, possibly because young birds deposited yolk at a slow rate. Indi vidual effects explained much of the variation in laying date and egg size. Egg size declined with laying date in a random sample of eggs la id by birds of unknown age. However, egg size was independent of layin g date among older birds. In contrast, egg size declined with laying d ate 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. The se results indicate that the lime-saving model does not adequately exp lain the seasonal decline in egg size in Thick-billed Murres. We sugge st that characteristics of individual Thick-billed Murres, largely med iated by age (or experience) during early reproductive attempts, deter mine the size of egg they lay.