Food availability and nest predation influence life history traits in Audouin's gull, Larus audouinii

The effects of food availability and nest predation on several life history traits such as adult survival, dispersal, and reproductive performance wer e assessed in an Audouin's gull (Larus audouinii) colony during the period 1992-1997. The amounts of fish discarded from trawlers were used as a measu re of food availability, and a trawling moratorium which partially overlapp ed with the breeding season of the gulls was taken into account. The effect s of nest predation were assessed in 1994, when a terrestrial predator ente red the colony and remained for the whole breeding season preying on both e ggs and chicks. Using the moratorium and the predatory event as natural exp eriments, several hypotheses were tested: (a) food supply would affect bree ding performance but not adult survival (independently of age and sex), sin ce gulls are long-lived and adult survival is the most sensitive demographi c parameter in their population dynamics; (b) the predator would trigger br eeding dispersal (although gulls are mostly philopatric, they are known to abandon their natal colony after breeding failure instigated by events such as this). If breeding dispersal occurs, the rate is expected to be higher in females than in males, and higher in new breeders than in more experienc ed breeding birds, as is usually recorded in colonial seabirds. Probabiliti es of resighting and survival were estimated separately, using capture-reca pture models, As expected, changes in food availability did not affect adul t survival, whereas they influenced egg volume, clutch size, and breeding s uccess. Local adult survival was estimated to be 0.908 (SD = 0.007) for mal es and females, and it did not change significantly with the age of individ uals (range 3-8 years). The predator significantly decreased breeding succe ss, and caused the dispersal of a number of adults probably to breed in ano ther colony; this rate was estimated at an average of 0.10 (SD = 0.02). As expected, inexperienced breeders dispersed significantly more (14%) than mo re experienced breeders (8%) after the predator event, but dispersal was no t sex biased. Recapture probabilities after the predator event suggest that birds that left the colony still had not returned. Results confirm that po pulation dynamics of ground-nesting seabirds are sensitive to terrestrial p redation, even when predation caused only a partial breeding failure.