Justification of Red List Category
This species has a highly restricted breeding range at a small number of colonies. Numbers breeding annually appear to fluctuate, but there is currently no evidence of a continuing decline, therefore the species has been reclassified as Near Threatened. Potential threats to breeding sites remain, and any evidence of declines would result in reclassification in a higher threat category.
The breeding population is variable between years, with the total number of nests estimated at 4,860 pairs in 2004, 7,790 pairs in 2007 and 5,240 in 2009 (Yorio et al. 2013), roughly equating to 15,600 mature individuals. The largest colony recorded is Isla del Puerto, with between 1,635 and 3,800 nests depending on the year (Petracci et al. 2008, Petracci and Sotelo 2013, Yorio et al. 2013).
The apparent population increase most likely reflects a more comprehensive simultaneous survey, and the variability between years in the observed total nesting population is over 20%, indicating the uncertainty involved and showing the difficulty in defining trends. Nevertheless, despite growing pressures at key breeding sites there is no current evidence that the population is declining.
Larus atlanticus breeds on the coast of Argentina between 38º49’ and 45°11'S (Yorio et al. 2005). It has been recorded breeding at eighteen coastal locations in the Bahía Blanca estuary and Bahía Anegada, south Buenos Aires, and north of Golfo San Jorge, Chubut, although there may be fewer than a dozen active nesting sites in any one year (Yorio et al. 1997, 1999 and 2013). The number of recorded colonies in each year has varied as it often changes breeding locations between years (Yorio et al. 1997 and 2013, García Borboroglu and Yorio 2007a). Over 98% of the total breeding population nests in southern Buenos Aires province and between 50-70% is concentrated in the Bahía Blanca estuary, an area subject to growing human development, disturbance, and pollution. In winter, the species disperses north, reaching Uruguay (Escalante 1970, Aspiroz 2003, Petracci and Sotelo 2013) and Brazil (Dias and Maurício 1998, Pacheco et al. 2009), with one inland record in Córdoba (Yzurieta 1995). The world population is currently estimated at 4,800-7,800 pairs (Yorio et al. 2013).
Behaviour Olrog's Gull is a coastal species and is a specialist predator, feeding mainly on crabs, which may be the result of interference competition with the larger and behaviourally dominant Kelp Gull (Larus dominicanus) (Delhey et al. 2001b). Olrog's Gull is spatially associated with the Kelp Gull throughout its breeding range (Yorio et al. 1997, García-Borboroglu and Yorio 2007a, 2007b). Olrog’s and Kelp gulls share several nesting microhabitat characteristics, so an increase in the area occupied by Kelp Gulls or in their nesting density as a result of their increase in numbers could affect the availability of adequate nesting habitats for Olrog’s Gulls (García-Borboroglu and Yorio 2007b). Kelp Gulls were also recorded preying upon Olrog’s Gull chicks at the Isla del Puerto colony (La Sala and Martorelli 2010).
Habitat It breeds on flat islands on bare ground or close to low vegetation, and close to the high tide line (Yorio et al. 2001, García Borboroglu and Yorio 2007a), and frequents beaches, rocky coasts, harbours, coastal and brackish lagoons and estuaries. Eggs are laid in September-October and chicks fledge in November or December (Yorio et al. 2005, La Sala et al. 2011).
Diet The species depends mainly on three crab species (Neohelice granulata, Cyrtograpsus altimanus and C. angulatus) during the breeding season (Delhey et al. 2001, Herrera et al. 2005, Petracci et al. 2007b, Suárez, et al. 2011). During the winter, it feeds also on crabs but shows a more opportunistic feeding ecology consuming molluscs, snails, small fish, insects, grains, garbage and fish discards (Spivak and Sanchez 1992, Martinez et al. 2000, Copello and Favero 2001, Delhey et al. 2001, Berón 2003, Silva Rodriguez et al. 2005, Berón et al. 2007, Petracci et al. 2007a, Berón et al. 2011, Seco Pon and Favero 2011, Berón et al. 2012). At Mar Chiquita Lagoon during the non-breeding season, some differences in diet were found between ages, particularly in spring when mature birds start to feed on the eggs of ovigerous females of Cyrtograpsus angulatus (Copello and Favero 2001).
Foraging range At Bahía San Blas, breeding Olrog’s Gulls gulls foraged mainly in three sectors located between 1.5 and 7 km north of the colony, although they can forage up to 30 km from the colony (Suárez et al. 2012, N. Suárez and P. Yorio, unpubl. data). At Golfo San Jorge, Argentina, feeding grounds of breeding Olrog’s Gulls were located up to 7 km from the colony, but 85% of mapped locations were within 3 km of the colony, almost exclusively within the Caleta Malaspina inlet (Yorio et al. 2004). The birds were consistent in the use of one particular area, and most birds foraged mainly in one or two areas (Yorio et al. 2004). Gulls foraged at all states of the tide, although they were more likely to forage during mid and low tides (Yorio et al. 2004). In Bahía Blanca estuary, recent studies conducted during the breeding season at the Isla del Puerto colony using GPS, indicated that adults use channels near the colony being able to roam up to 20 km from the colony, during the day and night (Petracci et al. unpubl. data). Non-breeding birds also show strong foraging site fidelity (Beron et al. 2007). When foraging, both during the breeding and non-breeding season, gulls are generally found along estuarine environments, brackish lagoons and open coastal areas with soft or rocky substrates, usually with presence of crabs (Escalante 1984, Favero et al. 2001, Silva Rodríguez et al. 2005, Yorio et al. 2004, Gatto et al. 2008, Suárez et al. 2012). In Argentina, it was observed to forage in crab-bed patches and beaches where sport-fishing activities usually took place (Beron et al. 2007). Individuals occasionally moved up to 4 km south to a village where they might also take advantage of rocky intertidal pools available along the shore (Beron et al. 2007). They have been also recorded in harbors, refuse tips, sewage outfalls or, rarely, at sea associated to fishing vessels (Martinez et al. 2000, Silva Rodríguez et al. 2005, Seco Pon and Favero 2011, Seco Pon et al. 2012, 2013). Similarly, in Mar Chiquita coastal lagoon, juveniles have often been observed making use of sport-fishing by-products (Copello and Favero 2001, Berón et al. 2007).
The Bahía Blanca estuary includes the city of Bahía Blanca, with over 400,000 inhabitants, and includes oil refineries, petrochemical industries, plastic factories, and pre-treated sewage outfalls all of which are important sources of pollutants (Marcovecchio 2000, Freije and Marcovecchio 2004). Recent studies at the Isla del Puerto colony have shown the potential negative effects of mercury pollution on breeding Olrog’s Gulls (La Sala et al. 2011). 90% of the breeding population of Olrog’s Gulls exists within the Bahía Blanca estuary and the Bahía San Blas Nature Reserve (Zalba et al. 2008). Olrog’s Gull is, broadly speaking, a diet specialist, preying primarily on crabs (Yorio et al. 2013). This increases its reliance on intertidal habitats and making it vulnerable to environmental modifications (Yorio et al. 2013). The main source of economic income in the Bahía San Blas Nature Reserve is sports fishing, this results in the discard of offal and other fish waste products which Olrog’s Gull has been found to eat in addition to its specialised crab diet (Berón et al. 2007, Berón et al. 2013, Martinez et al. 2000). This association has been known to result in the ingestion of hooks and entanglement in fishing line, with both scenarios associated with mortality of birds (Berón 2007, Berón and Favero 2009) resulting in an 56 deaths in the Mar Chiquita coastal lagoon between 2004-08 (Berón and Favero 2009).
The cultivation and collection of introduced Pacific Oysters may cause disturbance of nesting sites and foraging activity, as well as altering their habitat. Tourist activity may also disturb the gulls during breeding and they are known to abandon nesting efforts if disturbed (Zalba et al. 2008). In Uruguay many coastal sites where the species occurs in the non-breeding season have deteriorated due to the loss of adjacent wetlands through urbanisation and industrial pollution (D. Caballero-Sadi in litt. 2013), this is likely occurring across its range. Harvesting of eggs is thought to be relatively common in the Bahía San Blas colonies, despite being in a protected area.
Conservation Actions Underway
CMS Appendix I. All colonies in southern Buenos Aires, except Isla Brightman, are included in protected areas. The Bahía Blanca, Falsa and Verde Nature Reserve and the Bahía San Blas Nature Reserve protect twelve of the breeding locations reported in the last twenty years. Isla del Puerto was designated as a protected area in 2011 (Reserva Natural Integral Islote del Puerto; Provincial Decree 469). The colonies in southern Chubut gained legal protection in 2009 due to the designation of a new marine protected area in northern Golfo San Jorge, the Parque Interjurisdiccional Marino Costero Patagonia Austral (Law 26446) and they are also included in the recently designated UNESCO Biosphere Reserve Patagonia Azul (Garcia Borboroglu et al. 2015). In winter, it has been recorded in numerous reserves (Chebez et al. 1998, Chebez and Yorio 2008, A. B. Azpiroz in litt. 1999).
Conservation Actions Proposed
Given the frequent change in colony location of the species, develop comprehensive and simultaneous surveys throughout the breeding range during the same breeding season so as to obtain a correct estimation of the total breeding population (Yorio et al. 2005). Monitor the population (Yorio et al. 1997). Integrate management plans and actions through formal cooperative programmes (Yorio et al. 1999). Work with oil companies and governments to improve and enforce anti-pollution regulations (Yorio et al. 1999). Develop joint conservation actions between Argentina and Uruguay within the current framework of international agreements (Yorio et al. 2005). Evaluate natural and human related factors which may contribute to the observed changes between seasons in the use of colony sites (Yorio et al. 2005). Promote studies on the effects of pollution on populations in southern Buenos Aires (Yorio et al. 2005). Implement actions to eliminate egg harvesting and to minimize the negative impact of sport fisheries on non-breeding gulls. Implement monitoring programmes for the expanding Kelp Gull population and studies of behavioural interactions with Olrog's Gull (García Borboroglu and Yorio 2007b). Include non-breeding areas in conservation strategies, for example by creating areas to protect the species at the important Mar Chiquita, Buenos Aires province (Berón et al. 2007). Increased awareness and improved spatial protection may suggest that breeding populations are receiving adequate attention, but efforts are needed to ensure that guidelines for their protection are included in protected area management plans and that these are implemented effectively (Yorio et al. 2013).
50-56 cm. Large, black-and-white gull. White head, neck, rump and underparts. Rest of upperparts black. Black wings with white trailing edge. White tail with broad, black subterminal band. Yellowish legs. Yellow bill with black subterminal band and red tip. Juvenile mostly brown with dark mottling around head, dark tail and black bill tip. Similar spp. Kelp Gull L. dominicanus is slightly larger with wholly white tail, no subterminal band on bill and white tips to primaries. Voice Nasal and guttural calls.
Text account compilers
Sharpe, C.J., Stuart, A., Symes, A., Anderson, O., Harding, M., Hatchett, J., Benstead, P., Lascelles, B., Capper, D., Martin, R., Pilgrim, J.
Caballero Sadi, D., Petracci, P., Yorio, P., García Borboroglu, P., Azpiroz, A.
BirdLife International (2021) Species factsheet: Larus atlanticus. Downloaded from http://www.birdlife.org on 25/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 25/10/2021.