Diet and feeding behaviour of Olrog's Gull Larus atlanticus in Bahia Blanca, Argentina

Many gulls are mainly generalist feeders, but the Olrog's Gull Larus atlant icus is one of few gull species to which relatively specialised (carcinopha gous) feeding habits have been attributed. Endemic along the Atlantic coast of Argentina, Uruguay and southern Brazil, it is considered 'vulnerable' o wing to its apparent dietary specialisation, anthropogenic activities and s mall population. From October 1995 through September 1996 the diet and fora ging behaviour of this species was studied in the Bahia Blanca estuary, Pro vince of Buenos Aires, Argentina. The main prey was found to be crabs Chasm agnathus granulata and barnacles Balanus glandula, the latter during winter and the former throughout the rest of the year. This feeding behaviour cor relates with fluctuations in the abundance of crabs, which undergo a rest p eriod during winter, as against barnacles which are available throughout th e whole year. Although search and handling; times are significantly shorter for barnacles than for crabs, the latter are the preferred prey when avail able. The number of gulls increased during winter, possibly owing to the in flux of individuals from areas without barnacles, since fouling communities are scarce in the region and practically limited to solid substrates of an thropic origin. Interspecific aggression was observed only with Kelp Gulls Larus dominicanus, this latter species being dominant in 84% of the interac tions. Other food items (fish remains, bait, etc.) were of little significa nce and ingested only sporadically, their consumption almost always being i mpeded by Kelp Gulls. The apparent feeding specialisation of Olrog's Gulls could be due to the pressure of trophic interference competition by Kelp Gu lls in places where alternative food items (fishery waste) are scarce, as i s the case in the study area.