The Effect of Age on Timing of Breeding and Reproductive Success in the Thick‐Billed Murre

A seasonal decline in reproductive success is common to many birds breeding in seasonal environments. We examined the reproductive success of Thick—billed Murres breeding at Coats Island, Northwest Territories, Canada to assess the extent to which a seasonal decline in success could be accounted for by correlations between the age of the birds and their date of laying. In 1990 and 1991 reproductive success declined with date of laying: this decline was due mainly to a decline in the proportion of eggs that hatched. No corresponding decline was found in the proportion of hatchlings that reached fledging age. Pairs containing at least one young (4— or 5—yr—old) bird laid later and hatched a lower proportion of eggs than those containing at least one older (> 7—yr—old) bird. The two groups did not differ in the proportion of hatchlings reared. When looked at separately, the proportions of young and older birds that succeeded in hatching their egg were not affected by date of laying. When the two age groups were combined, the trends with date were similar to those found for a large sample, mostly birds of unknown age. To correct for differences in timing of laying, a sample of early breeders was induced to lay replacement eggs 14 d later. These experimental pairs were as successful as early control breeders. A greater proportion of the experimentally delayed pairs hatched chicks than birds laying for the first time at a similar date. The experimental pairs were also more successful than young birds that bred earlier. The growth of experimental and control chicks did not differ. However, the 14—d—old chicks of young pairs were lighter than those of experienced breeders, suggesting poorer provisioning by the young breeders. We concluded that there was no evidence for a deterioration in feeding conditions with date and that the relationship between age and/or experience and date of laying was the most important factor in causing the decline in reproductive success with date of laying was the most important factor in causing the decline in reproductive success with date. Our conclusions may apply to other studies of reproductive success in marine birds.