Social foraging behavior of the Olrog’s Gull (Larus atlanticus)

Different studies have analyzed the trophic behaviour of the Olrog´s Gull (Larus atlanticus). However, the sex and foraging group size effects have not been evaluated yet. During 2016 (non-breeding season) 171 focal samplings of marked individuals (n=20) were performed at the Mar Chiquita Reserve (37°46′ S, 57°27′ W). A total of 426 prey were registered (more than 80% of the items were identified as the burrowing crab Neohelice granulata). The crab’s size consumed differed from a uniform distribution (χ23=62.23, P<0.05); smallest prey were the most ingested (42% small, 20% medium, 22% large, and 14% very large). The effects of sex, age and group size were tested on the crab’s size consumed. None of the tested variables affected the proportion of “small” and “medium” consumed prey. Males individuals consumed more “large” prey than females and subadults more than juvenils (GLMM; t=2.98, P<0.05; t=1.82, P=0.08; respectively). Consumption of “very large” prey decreased with the increasing group size (t=-1.73, P=0.08). In relation with the evaluated trophic parameters: foraging effort (capture attempts by minutes) increased with the increasing group size (GLMM; t=2.31; P<0.05). This parameter, as well as the capture rate (prey captured by minutes) were affected by the identity of the individual (χ12=6.23, P<0.05; χ12= 9.86, P<0.05; respectively). This study provides novel information on the trophic ecology of a threatened species.