Summary 0[ 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 e}ort at younger age[ In contrast\ the {state!based| approach to life history postulates that not age per se determines reproductive decisions\ but the {state|\ re~ecting condition or quality that may di}er considerably between individuals of the same age[ In this respect\ we investigated the stability of body conditon\ as well as individual quality di}erences in common terns Sterna hirundo and outline the role of parental quality and parental e}ort in reproductive performance[ For lifetime identi_cation we marked terns with passive transponders[ Quality and e}ort were derived from body condition\ which we monitored during the whole breeding season and every year for a period of up to 4 years[ 1[ Parameters of body condition and reproduction varied signi_cantly between indi! viduals\ whereas intra!individual variation over years was very low[ Hence\ body condition is very stable and di}ers considerable between birds[ 2[ Among the factors number of chicks\ year and parental body condition\ the latter was the only signi_cant factor in~uencing 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 incu! bation and chick!rearing[ 3[ Parental e}ort increased signi_cantly with increasing breeding success[ The amount of expended e}ort depended on condition during incubation] mass loss was correlated with incubation mass\ but not with mass during chick!rearing[ 4[ Age was not an important factor in terms of condition or quality and also in terms of reproduction[ The age independent condition re~ects bird quality\ enabling high quality birds to invest high parental e}ort resulting in high reproductive success over years\ yet without negative consequences on future reproduction[ 5[ We conclude that in the common tern condition is a stable and age!independent individual feature with substantial e}ects on reproduction[ Hence\ our data strongly support the {state!based| approach to life history[ The high constancy in condition within and great di}erences between birds may also result in di}erences in lifetime reproductive success and _tness[ Key!words] body condition\ breeding success\ life history\ reproductive strategies\ state!dependence[ Journal of Animal Ecology "0888# 57\ 194Ð103

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In long-lived vertebrates, individuals generally visit potential breeding areas or populations during one or more seasons before reproducing for the first time. During these years of prospecting, they select a future breeding site, colony, or mate and improve various skills and their physical condition to meet the requirements of reproduction. One precondition of successful reproduction is arrival in time on the breeding grounds. Here, we study the intricate links among the date of initial spring arrival, body mass, sex, and the age of first breeding in the common tern Sterna hirundo, a long-lived migratory colonial seabird. The study is based on a unique, individual-based, long-term dataset of sexed birds, marked with transponders, which allow recording their individual arrival, overall attendance, and clutch initiation remotely and automatically year by year over the entire lifetime at the natal colony site. We show that the seasonal date of initial arrival at the breeding grounds predicts the individual age at first reproduction, which mostly occurs years later. Late first-time arrivals remain delayed birds throughout subsequent years. Our findings reveal that timing of arrival at the site of reproduction and timing of reproduction itself are coherent parameters of individual quality, which are linked with the prospects of the breeding career and may have consequences for fitness.prospecting ͉ recruitment age ͉ seabirds ͉ spring arrival

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