Effects of parental quality and effort on the reproduction of common terns

1. Age-related patterns of reproduction in birds are often interpreted in t erms of changing reproductive value with age: as residual reproductive valu e decreases with age birds have to increase their effort at younger age. In contrast. the 'state-based' approach to life history postulates that not a ge per se determines reproductive decisions, but the 'state', reflecting co ndition or quality that may differ considerably between individuals of the same age. In this respect, we investigated the stability of body conditon, as well as individual quality differences in common terns Sterna hirundo an d outline the role of parental quality and parental effort in reproductive performance. For lifetime identification we marked terns with passive trans ponders. Quality and effort were derived from body condition, which we moni tored during the whole breeding season and every year for a period of up to 5 years. 2. Parameters of body condition and reproduction varied significantly betwe en individuals, whereas intra-individual variation over years was very low. Hence, body condition is very stable and differs considerable between bird s. 3. Among the factors number of chicks, year and parental body condition, th e latter was the only significant factor influencing chick growth rates. Fu rthermore, high growth rates of chicks were correlated with good female con dition, and breeding success was positively correlated with body condition of both parents during incubation and chick-rearing. 4. Parental effort increased significantly with increasing breeding success . The amount of expended effort depended on condition during incubation: ma ss loss was correlated with incubation mass, but not with mass during chick -rearing. 5. Age was not an important factor in terms of condition or quality and als o in terms of reproduction. The age independent condition reflects bird qua lity, enabling high quality birds to invest high parental effort resulting in high reproductive success over years, yet without negative consequences on future reproduction. 6. We conclude that in the common tern condition is a stable and age-indepe ndent individual feature with substantial effects on reproduction. Hence. o ur data strongly support the 'state-based' approach to life history. The hi gh constancy in condition within and great differences between birds may al so result in differences in lifetime reproductive success and fitness.