Summary The Olrog’s Gull Larus atlanticus is an endemic and threatened species of the south-western Atlantic. Little is known about its movements during the non-breeding period. The objective of this study was to analyse the migration of the species by tracking adults from Bahía San Blas (Buenos Aires province, Argentina) with geolocators and using information of sightings of ringed gulls. Differences between males and females were evaluated using tracking data and ringed data were used to determine age differences. A single core area (kernel 50%) from 21 tracked birds was identified. This area included the study colony and also other breeding colonies located up to 300 km to the north. The range area (kernel 95%) included coastal areas up to 1,000 km from the colony. All sightings of ringed gulls (n = 41) occurred north of the breeding colony, however 12 adult individuals were sighted during the winter in its breeding grounds. Our results suggest the occurrence of partial migration behavior in Olrog’s Gull. The migration pattern reported here implies than during the non-breeding season, breeding and wintering areas away from the nesting grounds should be considered as one system in the design of conservation strategies for this regionally threatened gull.

SummaryOlrog’s Gull Larus atlanticus is an endemic species of the Atlantic coast of southern South America and is currently listed as ‘Near Threatened’ on the IUCN Red List. The species breeds in coastal wetlands of Buenos Aires and Chubut provinces of Argentina and during winter migrates northwards as far as southern Brazil. During the breeding season the species is considered to have a specialised diet, foraging mostly on crabs. However, during the non-breeding season the gull presents a wider dietary spectrum (including fish) as a result of a potential association with commercial and sport fishing activities. The main goal of this study was to analyse the habitat use and overlap with natural and anthropogenic food resources of juvenile Olrog’s Gull during the winter in Mar Chiquita Lagoon, Buenos Aires. Twenty-two GPS devices were deployed during the winters of 2013 and 2014. A total of 1,088 fixes was gathered by GSM (both seasons combined). Analysis showed that the individuals were (kernel 95%) distributed in a limited area along the mouth of the lagoon. The core area (kernel 50%) covered only 0.3 km2 and was located in the south of the mouth. The overlap between juveniles and crab densities was lower than expected, while the overlap between juveniles and fishing effort was higher than expected during weekdays and lower during weekends. These results are of value for the further development of marine conservation measures for wintering areas which have so far been somewhat neglected while addressing the conservation status of the species.