Age effects on reproductive success, nest-site location, and offspring condition in the Great Cormorant Phalacrocorax carbo sinensis

Abstract

Breeding performance of many vertebrate species is known to improve over the early stages of the life cycle, which has been commonly attributed to the progressive improvement in competences and increasing investment in reproduction. While there is a large body of evidence for age-related variation in fecundity within bird populations, much less is known on whether and how age affects other traits related to reproduction. The aim of this study was to test for age-related patterns in fecundity, nest-site location, offspring condition and brood sex ratio in a long-lived colonial waterbird, the Great Cormorant (Phalacrocorax carbo sinensis). For this purpose we collected data on 180 breeding attempts of 100 Great Cormorants aged two to 11 years. We found a progressive increase in reproductive output with age and we showed that older individuals nested closer to the colony centre and in higher nesting densities. We also found that parental age was positively related to physiological condition of offspring (measured with whole-blood haemoglobin concentration and plasma glucose concentration). There was no indication for age-related adjustments in brood sex ratio in our cormorant population. Our results indicate a great complexity of the processes which produce age-related improvements in reproductive performance within avian populations.