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Age-Related Differences in Piracy Behaviour of Four Species of Gulls, Larus

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The authors studied the piracy behavior of four species of Larus gulls in New Jersey and Mexico to learn more about the effects of age on piracy behavior. Piracy refers to the stealing of food from another individual and usually involves aerial chasing. For the four species observed feeding at a garbage dump: adults engaged in piracy more often than young gulls, there were no differences in piracy success rates, adult herring and ring-billed gulls were chased less than young of these species but there were no age-related differences for the other two species, and only herring gulls showed an age difference in victim rates with adults being victimized less often than young. Herring gulls were the most frequent pirates although all species were both pirates and victims. There were few piracy attempts on great black-backed gulls. Several measures of piracy behavior improved from September to February in herring gulls: the percentage of food dropped decreased, the victim rate decreased, and victim losses decreased. Ring-billed gulls pirating from starlings foraging on the ground near a dump showed age differences in piracy success for different methods. When provisioned with pre-cut pieces of liver in Mexico, laughing gull adults were more successful than subadults and young. Young were unable to obtain and keep larger pieces. Similarly, young gulls pirated from young rather than from adults. Taken altogether, this study indicates differences in piracy rates, piracy success rates, victim rates and victim losses as a function of age and methods used by some species. Improvement occurred with age and experience.

Affiliations: 1: Dept. of Biology, Rutgers University, and Dept. of Environmental & Community Medicine, Rutgers Medical School, New Brunswick and Piscataway, N.J., U.S.A.


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