FEEDING HABITAT SELECTION IN THE THREATENED OLROG'S GULL LARUS ATLANTICUS BREEDING IN NORTHERN PATAGONIA, ARGENTINA

SUÁREZ, NICOLÁS; KASINSKY, TATIANA; RETANA, VALERIA; MARINAO, CRISTIAN; YORIO, PABLO

Cape Town

Congreso; II World Seabirds Conference; 2015

World Seabird Union

The Olrog´s Gull Larus atlanticus is a threatened species which feeds during the breeding season almost exclusively on three intertidal crab species. We studied the feeding habitat selection during the incubation and early chick stage of gulls breeding at two colonies (Islote Jabalí and Banco Nordeste) located in the Bahía San Blas marine protected area, Argentina. Feeding areas were identified by GPS tracking of 67 birds (414 foraging trips). Potential feeding areas were classified using dominant substrate and structural characteristics and prey characteristics were assessed by randomly sampling 3,638 1 m2 quadrates. Habitat classification was conducted using Landsat TM satellite imagery, complemented by a supervised classification using 80 randomly located reference plots in the field. The patterns of feeding habitat selection throughout the breeding season differed between the two colonies. Birds from Islote Jabalí selected habitats characterized by muddy substrates with vegetation and high densities of Neohelice granulata during incubation, but switched to structured habitats (mainly oysters beds) dominated by the smaller Cyrtograpsus altimanus. In contrast, birds from Banco Nordeste selected habitats with muddy substrates throughout the breeding cycle. Crab size differed between foraging areas used by birds from the two colonies, being the sizes of N. granulata significantly smaller in the Banco Nordeste foraging grounds. Previous studies showed that prey delivered to chicks were significantly smaller than those use by adults for self-feeding. The selection of coastal habitat by Olrog´s Gulls appears to be determined by the seasonal change in trophic requirements of adults and chicks, given the spatial distribution of prey sizes.