Proximate and ultimate constraints on breeding in seabirds

Life-history trade-off theory states that long-lived species, such as seabirds, respond to sub-standard environmental conditions by reducing annual investment in reproduction to ensure a high probability of survival. Thus, constraints on reproductive investment are defined by threshold levels of investment needed for survival.; I used the hormone corticosterone to determine when adult body condition fell below the threshold necessary to sustain parental investment in Caspian Terns (Sterna caspia) breeding in the Columbia River estuary of Oregon, USA. Elevated corticosterone levels are associated with decreased reproductive investment. Only when low body condition was associated with elevated corticosterone was low body condition considered below threshold. Breeders maintained a higher body condition threshold in the poor food year (2000) than in the good food year (2001), indicating they increased energetic buffering when food was less available. The body condition threshold was higher during incubation than late chick rearing, following a predictable decline in body mass during the breeding season. Thus, Caspian Terns prepare for both predictable and stochastic declines in body condition by raising the threshold body mass prior to declines.; Albatrosses do not replace all their wing flight feathers every year, and the extent of molt is limited by time spent breeding. In Laysan ( Phoebastria immutablis) and Black-footed (P. nigripes) Albatrosses the number of outer primary feathers replaced annually (3--5), and the number of "molt series" that initiate molt each year (2--4) are associated with molt initiation date and molt duration. Medium and small molts do not appear to reduce reproductive investment, but large molts do. A medium-sized molt is an adequate annual molt. Consequently, I propose that breeders that undertake medium-sized molts year after year do not experience trade-offs and can breed annually, whereas breeders that undertake a small molt one year, and a large molt in a subsequent year experience trade-offs between molt and breeding, and must skip a year of breeding.