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

Abstract

Olrog's Gull Larus atlanticus is a threatened species which feeds during the breeding season almost exclusively on crabs.

Temporal Changes in Diet and Prey Selection in the Threatened Olrog's GullLarus atlanticusBreeding in Southern Buenos Aires, Argentina

“…1). This sector is used as foraging grounds by Olrog's Gulls nesting at Islote Arroyo Jabalí Oeste (40° 32' 42.20" S, 62° 17' 23.48" W) (Suárez et al 2012). Colony size during the study year was estimated at 508 nests (N. Suárez, unpubl.…”

“…C. altimanus is mostly found in structured environments, while N. granulata is found in marshes characterized by muddy substrates with Spartina spp. and C. angulatus in sectors dominated by muddy substrates (Iribarne et al 2003;Isacch et al 2006;Suárez et al 2012). Olrog's Gulls in our study area feed almost exclusively (99% of total prey biomass) on the three crab species, primarily N. granulata and C. altimanus, with a differential contribution of each species depending on the stage of the breeding cycle (Suárez et al 2011).…”

“…Observations were made on feeding adult individuals distributed throughout the entire coastal area used for foraging (Suárez et al 2012). From eight to 15 focal individuals were selected randomly and observed daily from an estimated distance of 50 m. Each feeding Olrog's Gull was filmed for 5 min using a digital camcorder (Sony Handycam SR68, with 60X optical), and recordings were then analyzed in the lab to obtain feeding technique and feeding rate.…”

“…Olrog's Gull (Larus atlanticus) is an endemic species of the southwestern Atlantic coast that only breeds in Argentina (Yorio et al 1999) and is considered globally vulnerable (Birdlife International 2013). During the breeding season, Olrog's Gulls prey almost exclusively on intertidal crabs (Delhey et al 2001;Herrera et al 2005;Suárez et al 2012) and thus are highly dependent on intertidal environments. Like other gulls, Olrog's Gulls use a variety of feeding techniques, such as pecking, head submergence, and surface plunging, allowing capture of prey both in exposed and covered intertidal areas (Martinez et al 2000;Copello and Favero 2001;Gatto et al 2008), and studies during the non-breeding season have revealed differences in feeding techniques (Copello and Favero 2001) and capture rates (Berón et al 2011) among different age-classes.…”

Use of winter habitat at an early age: spatial ecology and association with human activities of juvenile Olrog’s GullLarus atlanticus

“…For N. granulata, an inter-tidal burrowing crab species, 39 stratified random plots measuring 50 x 50 cm were placed in different environments previously characterised in Isacch et al (2006) and sampled during 18-22 July 2014 ( Figure S1). In each quadrat, the number of burrows was counted as a proxy indicator of crab abundance (Suárez et al 2012). For the sub-tidal C. angulatus, seven transects were performed during low tide along the edge of the estuary during 22-25 July 2014 ( Figure S1).…”