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

Abstract

1. Age‐related patterns of reproduction in birds are often interpreted in terms of changing reproductive value with age: as residual reproductive value decreases with age birds have to increase their effort at younger age. In contrast, the ‘state‐based’ approach to life history postulates that not age per se determines reproductive decisions, but the ‘state’, reflecting condition 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 and outline the role of parental quality and parental effort in reproductive performance. For lifetime identification we marked terns with passive transponders. Quality and effort were derived from body condition, which we monitored 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 between individuals, whereas intra‐individual variation over years was very low. Hence, body condition is very stable and differs considerable between birds. 3. Among the factors number of chicks, year and parental body condition, the latter was the only significant factor influencing chick growth rates. Furthermore, high growth rates of chicks were correlated with good female condition, 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: mass 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 also in terms of reproduction. The age independent condition reflects bird quality, 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‐independent individual feature with substantial effects on reproduction. Hence, our data strongly support the ‘state‐based’ approach to life history. The high constancy in condition within and great differences between birds may also result in differences in lifetime reproductive success and fitness.