SURVIVAL COSTS OF CHICK REARING IN BLACK-LEGGED KITTIWAKES

1. We tested for costs of chick rearing in the black-legged kittiwake Rissa tridactyla (Linnaeus) by removing entire clutches from 149 of 40 5 randomly selected nests, in which one or both mates was colour-bande d. After the manipulation, we monitored adult nest attendance and body condition at unmanipulated and manipulated nests, and measured the su rvival and fecundity of these adults the following year. 2. Late in th e chick-rearing period, adults from unmanipulated nests (i.e. with chi cks) went on significantly longer foraging trips, and were significant ly lighter for their size, than adults from manipulated nests (i.e. wi thout chicks). 3. Adults from unmanipulated nests also survived to the following nesting season at a significantly lower rate than those fro m the manipulated nests (0.898 vs. 0.953), suggesting that attempting to raise chicks can reduce life expectancy by 55%. 4. There was a tend ency for adults from nests that were unmanipulated in year one to have lower reproductive success in year two, primarily because of reduced hedging success, and a higher incidence of non-breeding. 5. These find ings suggest that mass loss in kittiwakes during chick rearing may not be adaptive. Raising chicks can lead to reproductive costs, and the c ausal mechanism appears to be a reduction in body condition. 6. We com pare our results with previous brood (or clutch) size manipulation exp eriments that have measured adult body condition, survival and/or futu re fecundity. Although the empirical evidence suggests that long-lived species are more likely to experience survival costs than short-lived species, we believe the opposite may be true. We suggest that shiftin g the experimental protocol of cost of reproduction studies from brood enlargements (an approach taken in most prior studies) to brood reduc tions will provide more accurate quantifications of naturally occurrin g costs. 7. The cost of reproduction is one mechanism proposed to expl ain the reduced survival rates reported for kittiwake populations in t he North Atlantic relative to those in the North Pacific ocean. Oceano graphic data, however, suggest that lower food availability may limit survival of kittiwakes in the North Atlantic where a deeper mixed laye r and reduced primary production combine to make conditions less favou rable for this seabird during the winter months.