Spatial patterns in the use of foraging areas and its relationship with prey resources in the threatened Olrog's Gull (Larus atlanticus)

Olrog's Gull Larus atlanticus is a threatened species which feeds during the breeding season almost exclusively on crabs. We studied the use of foraging areas and its relationship with prey resources in Olrog's Gulls breeding at Bahia San Blas, Argentina. Feeding areas were identified by radio-tracking 10 and 12 birds in 2006 and 2007, respectively (458 and 574 foraging trips, respectively) and monitoring 120 color-marked individuals during 2007 (3,447 locations). Feeding habitats were classified using dominant substrate and structural characteristics, and prey availability was assessed by sampling 2,220 1-m(2) quadrates distributed throughout gull potential feeding areas. Both telemetry and monitoring of marked individuals indicated that gulls used 20 km of coastline but foraged mainly in three sectors located between 1.5 and 7 km north of the colony. During both years, the use of feeding areas varied throughout the breeding cycle, with a higher use of areas closer to the colony during the chick stage. Results showed a differential distribution of crab species depending on habitat type, with a dominance of Cyrtograpsus altimanus in structured environments and Neohelice granulata in muddy substrates with vegetation. During incubation, gulls mostly used areas characterized by high densities of N. granulata, while during the early chick stage they mostly used sectors with high densities of C. altimanus. Prey size varied among crab species, C. altimanus being significantly smaller. Changes in Olrog's Gull use of coastal areas appear to be determined by the seasonal change in trophic requirements of adults and chicks, given the spatial segregation of their prey in relation to habitat characteristics.