Estimating predation on breeding European storm-petrels (Hydrobates pelagicus) by yellow-legged gulls (Larus michahellis)

Abstract

Several hypotheses about changes in adult survival of European storm-petrels Hydrobates pelagicus and predation by yellow-legged gulls Larus michahellis at Benidorm Island (western Mediterranean) during 1993–2003 were assessed. Two approaches were used: analysis of pellets containing remains of predated petrels from two caves, and multistate capture–recapture models using recaptures of breeders together with recoveries only from one of these caves. These models were used to estimate separately adult storm-petrel survival and a minimum probability of being killed by gulls. At least 11–14% of the storm-petrels killed had been ringed as breeders. Local adult survival varied greatly among years, from 0.701 (SE=0.095) to extremely high values (0.951, SE=0.031) for such a small species (0.833 on average for the whole period). Nevertheless, it was not possible to assess whether this variation was influenced by inter-annual differences in gull predation. The peak of predation occurred in June, when maximum numbers of prospecting petrels visit the colonies. Predation was much higher at the cave that received stronger illumination from Benidorm city. Minimum predation probabilities estimated from capture–recapture varied with time from 0.022 (SE=0.011) to 0.040 (SE=0.016), and increased significantly after the powering of light received from the city in 1997. In these years, predation by yellow-legged gulls accounted for a large percentage of mortality of storm-petrels at Benidorm Island (up to 33%). Both methodological approaches confirmed that predation was carried out by specialist gulls, and that neither the size of the gull colony nor its food availability influenced predation on petrels. Management might reduce predation rate by removing gulls that nest in the caves, since these individuals tend to kill most storm-petrels.