Parental care of replacement clutches in common terns (Sterna hirundo)

In long-lived seabirds with low annual reproductive output, the renesting d ecision after breeding failure is critical, and the parents have to weigh b enefits of replacement clutches against possible future reproductive costs. In this study, we investigated factors influencing renesting decisions in common terns (Sterna hirundo) and compared aspects of breeding biology and body mass between two breeding attempts by the same pairs in each of 4 year s of heavy losses due to predation. Renesting birds were characterized by e arly laying dates and by a high age. Among early breeders, high egg mass re duced the probability of renesting. A long relaying interval coincided with low mass of replacement eggs in one year, and short intervals with high eg g mass in another. Further, egg mass decreased and relaying intervals incre ased the later the predation events occurred. Evidence of high levels of pa rental care of replacement clutches came from body mass data: female mass i ncrease prior to egg laying was higher in the second attempt than in the fi rst, whereas male mass was lower during the second courtship period than du ring the first. Male mass also affected relaying intervals and mass of repl acement eggs. We conclude that common terns expend high levels of parental care of replacement clutches. Intrinsic factors related to individual quali ty (age, body condition) seemed most important for renesting decisions and for the degree of parental care, but foraging conditions seemed to have mod ifying effects.