Distribution of Olrog’s GullLarus atlanticusfrom Bahía San Blas during the non-breeding period: signals of partial migration

Abstract

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.

Chlorpyrifos and persistent organic pollutants in feathers of the near threatened Olrog’s Gull in southeastern Buenos Aires Province, Argentina

“…Given that a high proportion of the species breed in southern Buenos Aires Province, it is highly likely that at least part of the non-breeding population wintering in Mar Chiquita comes from this area. Besides, a recent study indicates that birds breeding in more southern latitudes like Bahía San Blas, shows signs of partial migration behavior during the non-breeding season (Copello et al, 2020). Thus, our results suggest that juvenile birds probably reflect the compounds they have incorporated during the breeding season.…”

“…2). The breeding distribution of the Olrog's gull is chiefly restricted to southern Buenos Aires Province, with northward post-breeding migration movements along coastal areas of northern Buenos Aires Province, Uruguay and southern Brazil, and part of the population remaining in the vicinity of breeding grounds (Copello et al, 2020). The most likely explanation related to the observed difference between juvenile and adult birds is that juvenile birds found in the Mar Chiquita coastal lagoon are likely to include individuals that recently abandoned their breeding grounds (see Fig.…”