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Distinguishing Between Specialism and Opportunism in the Diet of Herring Gulls

Laura Post

出版	Pennsylvania State University, 2017
URL	http://books.google.com.hk/books?id=GZldtAEACAAJ&hl=&source=gbs_api

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For large gulls, predation of other waterbirds is generally considered a specialist strategy and may have potential benefits for reproductive success. During three years of study, however, nearly all herring gulls (Larus argentatus) breeding around the edges of a large ring-billed gull (L. delawarensis) colony on Gull Island, Ontario, were found to eat the chicks of this smaller species to some extent. I collected diet, behavioral, and productivity data to differentiate the extent to which predatory behavior was a specialist strategy or merely opportunistic, test the hypothesis that feeding on waterbirds is evolutionarily beneficial, and evaluate current management strategies in light of the results. Herring gulls predated ring-billed gull chicks most frequently when these chicks were most readily available: immediately after hatching and then again, around fledging when they formed crches on the beach. Herring gulls holding territories close to ring-billed gull crches exploited ring-billed gull prey to a greater extent. Behavioral watches and over 1,900 hours of remote video footage (both day and night) showed that all attacks were initiated within the herring gulls own breeding territories and predatory attacks did not include any specialized behavioral adaptations. Lastly, ring-billed gull chicks appeared to be a relatively poor food item for herring gull chicks and there was no consistent relationship between herring gull productivity and the proportion of ring-billed gull chicks in the diet. I conclude that at this site waterbird predation by herring gulls is predominantly opportunistic and does not appear to show any immediate benefit in terms of evolutionary fitness. This indicates that current management strategies based on controlling specialist predatory gulls are likely to be ineffectual in mixed-species colonies such as this and could be replaced by effective, non-lethal management, such as physical barriers.