Ardea
Official journal of the Netherlands Ornithologists' Union

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Delhey J.K., Carrete M. & Martinez M.M. (2001) Diet and feeding behavior of Olrog's Gull Larus atlanticus in Bahia Blanca, Argentina. ARDEA 89 (2): 319-329
Many gulls are mainly generalist feeders, but the Olrog's Gull Larus atlanticus is one of few gull species to which relatively specialised (carcinophagous) feeding habits have been attributed. Endemic along the Atlantic coast of Argentina, Uruguay and southern Brazil, it is considered 'vulnerable' owing to its apparent dietary specialisation, anthropogenic activities and small population. From October 1995 through September 1996 the diet and foraging behaviour of this species was studied in the Bahia Blanca estuary, Province of Buenos Aires, Argentina. The main prey was found to be crabs Chasmagnathus granulata and barnacles Balanus glandula, the latter during winter and the former throughout the rest of the year. This feeding behaviour correlates with fluctuations in the abundance of crabs, which undergo a rest period during winter, as against barnacles which are available throughout the whole year. Although search and handling times are significantly shorter for barnacles than for crabs, the latter are the preferred prey when available. The number of gulls increased during winter, possibly owing to the influx of individuals from areas without barnacles, since fouling communities are scarce in the region and practically limited to solid substrates of anthropic origin. Interspecific aggression was observed only with Kelp Gulls Larus dominicanus, this latter species being dominant in 84% of the interactions. Other food items (fish remains, bait, etc.) were of little significance and ingested only sporadically, their consumption almost always being impeded by Kelp Gulls. The apparent feeding specialisation of Olrog's Gulls could be due to the pressure of trophic interference competition by Kelp Gulls in places where alternative food items (fishery waste) are scarce, as is the case in the study area


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