Senescence and reproductive performance in the European shag (Phalacrocorax aristotelis)

Senescence(definition) is rarely observed in wild populations. This is due to a combination of (a) high extrinsic mortality rates, for instance acting through predation or food limitation, which mean wild animals rarely die of old age, (b) the inevitably small numbers of old individuals, which limits sample size, and (c) the difficulties of studying known age individuals across appropriate time periods. Motivated by reports of lower survival in old European shags Phalacrocorax aristoteiis, this project aimed to determine whether reproductive senescence or declines in adult condition also occur in old birds of this species breeding on the Isle of May, Scotland. A large proportion of the shags at this site are known age as a consequence of long-term ringing effort. Telomere(definition) length, which plays a role in cellular senescence and organismal ageing, was also measured among birds of different age, and in serial samples taken from the same individuals, in order to assess factors affecting rates of telomere attrition. The correlation between the ages of breeding partners was found to be weak in old shags, and the age difference in pairs increased in relation to the age of the female. Therefore, in the oldest shags the age of one pair member does not provide a reliable estimate of the age of its partner. Old female age was confounded with young male age in pairs, and thus with relatively newly established partnerships. A pattern of smaller eggs laid by old females was attributed to declining egg size within individuals. However, at least on a population level, this did not translate into lower hatching or fledging success, and old shags do not fledge fewer chicks than middle- aged birds. There was a tendency for complete breeding failure to be associated with a large age difference between breeding partners. There was no difference in the quality of the nest site occupied by old shags, and old birds also did not commence breeding later than expected. With the exception of a decline in number, and increase in size, of red blood cells, no differences in the physical attributes of adult shags were observed in old age, despite adequate samples of old individuals. This may in part reflect biases in the sample of birds that could be measured, and the years in which the study was carried out. Telomere length declined within individuals between the nestling and adult stage. However, in adults there was no relationship between age and telomere length. Environmental conditions experienced as a chick were related to subsequent telomere shortening in these birds, suggesting that early conditions, possibly through effects on oxidative stress, may play a role in telomere attrition, and potentially in the longevity of individuals.