Gull Larus Atlanticus Research Articles

Effect of urbanization on individual condition of a threatened seabird: the Olrog’s Gull Larus atlanticus

Effect of urbanization on individual condition of a threatened seabird: the Olrog’s Gull Larus atlanticus

Where are you from? Isotopic tracing of juvenile Olrog’s Gulls from Mar Chiquita during the wintering season

ABSTRACT Individual traits such as age-class can have profound effects on space utilisation by individual birds, with juvenile birds more often exhibiting dispersive movements. We studied the origin of juvenile Olrog’s Gulls Larus atlanticus, a threatened species, using stable isotopes ratios of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S). We analysed feathers from individuals captured in the wintering season at Mar Chiquita Coastal Lagoon, Argentina, and the potential trophic sources of their breeding sites at Bahía Blanca and San Blas. Mean δ13C was similar for trophic sources and mean δ15N was higher for Bahía Blanca. The δ34S showed a greater variation among prey species than among sites. The mean contributions of the main colonies resulted in assigning 37.5% individuals as from Bahía Blanca, 16.7% from San Blas, and 45.8% undetermined. Results indicate that Mar Chiquita coastal lagoon attracts juvenile Olrog’s Gulls dispersing from the two main breeding grounds of the species. This work is one of the first approaches to study the origin of dispersive juvenile larids through the application of stable isotope analysis. This isotopic approach allows obtaining trophic and movement information when it is not possible to use other tools.

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-m2 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.

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

Fil: Suarez, Nicolas. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Nacional Patagonico; Argentina

Some Digeneans from Olrog's Gull Larus atlanticus Olrog, 1958 (Aves: Laridae) from the Bahía Blanca Estuary, Argentina

Five species of digeneans (4 Microphallidae, 1 Echinostomatidae) were recovered from a total of 85 Olrog's gulls, Larus atlanticus (dead chicks). Birds were collected from a breeding colony in the Bahía Blanca estuary, Argentina, during breeding seasons 2003 (n = 6), 2005 (n = 66), and 2006 (n = 13). Digeneans were identified as Maritrema bonaerensis, Maritrema orensensis, Odhneria odhneri, Levinseniella cruzi, and Himasthla escamosa. Except for M. bonaerensis, all other helminth species represent new host records, and all the digeneans reported constitute new locality records. Likely intermediate hosts for digeneans, such as Neohelice granulata (Crustacea: Decapoda), Cyrtograpsus angulatus (Crustacea: Decapoda), and Heleobia australis (Mollusca: Gastropoda), were also studied. The metacercariae of M. bonaerensis were found in the crabs N. granulata and C. angulatus, and cercariae closely resembling those of M. bonaerensis, L. cruzi and H. escamosa were found in the mollusc H. australis.

Diet specialization of Olrog's Gull Larus atlanticus during the breeding season at Golfo San Jorge, Argentina

We studied the diet of breeding Olrog's Gulls Larus atlanticus during 1994 at two colonies in Golfo San Jorge, Argentina, through the analysis of 207 regurgitated pellets. The diet at Isla Felipe and Isla Vernaci Sudoeste comprised only a few items (9 vs 7 respectively), of which crabs were the main component. At Isla Felipe, the crabs Cyrtograpsus altimanus and C. angulatus were present in 91.2% and 89.2% of pellets, respectively, while the percentage contribution of each species was 76.1% and 21.7%, respectively. At Isla Vernaci Sudoeste, the figures were similar (96.2% and 80% of pellets, and percentage contributions 79.4% and 19.5%). All other taxa were present in less than 11% of pellets. No significant differences were found between colonies in the number of prey items, prey individuals per pellet, and consumption of crabs. Percentage contribution of crabs differed significantly between stages of the breeding cycle at Isla Felipe but not at Isla Vernaci Sudoeste. At both colonies, the proportion of male C. altimanus in pellets was larger than that of females. The proportion of female C. angulatus in pellets at Isla Vernaci Sudoeste was larger than that of males, but no significant differences were found in the consumption of each sex at Isla Felipe. Mean maximum carapace width of male crabs in pellets was not significantly different from that of females at either Isla Felipe or Isla Vernaci. Our study at Golfo San Jorge confirms that Olrog's Gull is fairly specialized, feeding mainly on crabs during the breeding season.

Foraging ecology of Olrog's Gull Larus atlanticus in Mar Chiquita Lagoon (Buenos Aires, Argentina): are there age-related differences?

Foraging ecology of Olrog's Gull Larus atlanticus in Mar Chiquita Lagoon (Buenos Aires, Argentina): are there age-related differences?

Habitat and nest site characteristics of Olrog's Gull Larus atlanticus breeding at Bahía San Blas, Argentina

Olrog's Gull Larus atlanticus is a vulnerable species endemic to the Argentine Atlantic coast. We present information on new breeding colonies, update information on known colonies, and describe habitat and nest site characteristics of Olrog's Gulls breeding at Bahía San Blas, southern Buenos Aires Province, Argentina. Information was obtained during the 1998 breeding season. We recorded four colonies on islets, with a total population size of 305 pairs. Colonies were located on open ground, partly bordered by vegetation. The two colonies at Islote Arroyo Jabalí Oeste were located more than 150 m from the high tide line, while the two colonies at Banco Nordeste were placed only a few metres from the high tide mark (1-6 m). Most nests were located more than a metre away from low Atriplex patagonica bushes (20-30 cm high) and herbaceous species, although a few nests were a few cm from vegetation. Nests were built on vegetation debris, on low herbaceous vegetation, and on gravel and sand. All nests at the four colonies were built with vegetation debris, mainly Spartina densiflora and some Salicornia sp. Mean external and internal diameters of nests were 40.57 ± 5.14 and 19.71 ± 2.21 cm, respectively. Gulls nested in discrete groups at relatively high densities, which ranged between 0.62 and 1.1 nests/m2. Nest occupation and number of eggs per nest varied among colonies. Kelp Gulls Larus dominicanus were observed nesting in close proximity to all colonies. Further studies are needed to assess habitat partitioning between these two species, particularly considering that they nest in association at all breeding locations in coastal Argentina and that Kelp Gulls have shown an important population increase in recent decades.

Olrog's Gull Larus atlanticus: specialist or generalist?

A study was made of the feeding behaviour of Olrog's gull Larus atlanticus in winter. Although feeding mainly on crabs they also, contrary to previous observations, made use of discarded offal, much as other species of gull. Such generalist feeding habits are likely to be beneficial to the conservation of the species.