Increased adult mortality and reduced breeding success with age in a population of common guillemot Uria aalge using marked birds of unknown age

Abstract

Although there is general consensus about the existence of senescence in vertebrates, empirical evidence of senescence in demographic parameters in wild populations is limited. Data on breeding success and survival of breeding common guillemots Uria aalge were collected over 20 years on the Isle of May (Scotland) using a pool of individuals marked as adults. Because the years of hatching of individuals were not known, we used the time (years) elapsed since first capture (TFC) as a measure of age. The use of this proxy did not create any bias in estimating senescence in the case of a linear decline, nor did it greatly decrease the power of a test for senescence. Breeding success declined significantly from 0.81 (95% CI: 0.77–0.84) to 0.62 (0.54–0.68) over the study period. It also varied in relation to age, initially increasing (from 0.62 (0.54–0.68) at TFC of 0 year to 0.76 (0.73–0.79) at TFC of 9 years) up to a plateau (from TFC of 9 years with 0.76 (0.73–0.79) until TFC of 13 years with 0.77 (0.74–0.79) before declining in later life to 0.70 (0.61–0.78) at TFC of 19 years). Survival was generally high and varied significantly from year to year. It also declined with TFC: survival of birds marked in 1982 decreased from 0.92 (0.85–0.96) at a TFC of 0 year to 0.88 (0.82–0.92) at a TFC of 19 years. Resighting probabilities also declined with TFC suggesting that the oldest birds do not come back to the colony to breed as regularly as younger individuals. These findings indicate that individual common guillemots on the Isle of May showed both actuarial and reproductive senescence.