Diet composition and breeding success in a threatened Pacific seabird, the marbled murrelet (Brachyramphus marmoratus)

Poor reproductive success due to changes in feeding conditions at sea has been identified as a potential cause of range-wide declines in abundance of marbled murrelets, but the mechanisms involved remain poorly understood. To evaluate two hypothesized mechanisms by which diet quality might influence reproductive success, murrelets and their prey were captured during the breeding seasons of 2007 and 2008 in Desolation Sound, BC. Vitellogenin from blood plasma of murrelets was used to estimate timing of breeding, and stable-carbon and -nitrogen isotopes from murrelet and prey tissues were used to estimate diet composition. Contrary to predictions of the hypothesis that pre-breeding trophic feeding level positively influences female body condition and timing of breeding, adult females feeding at a lower trophic level prior to breeding in 2007 were in better condition and more likely to produce an egg early in the season, but this result was not consistent among years. In accordance with predictions of the hypothesis that diet quality during development positively influences juvenile growth and body condition, juveniles feeding on higher quality prey were larger and in better condition. These results suggest that feeding conditions may influence reproductive success of marbled murrelets through effects on both adults and juveniles.