BirdLife species factsheet for Olrog’s Gull Olrog’s Gull Larus atlanticus breeds on the coast of Argentina between 38°49′ and 45°11’S (Yorio et al. 2005). It is currently listed as Vulnerable under B1ab(ii,iii,v); C2a(i) because it has a small range and population (<10,000 mature individuals), both of which were thought likely to be undergoing a slow, continuing decline owing to a variety of threats to its breeding grounds, and all subpopulations were thought to contain less than 1,000 mature individuals. 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 (Yorio et al. 1997, Yorio et al. 1999, P Yorio, P. García Borboroglu, and P. Petracci, unpubl. data). The number of recorded colonies in each year has varied as it often changes breeding locations between years (Yorio et al. 1997, García Borboroglu and Yorio 2007, P Yorio, P. García Borboroglu, and P. Petracci, unpubl. data). Over 98% of the total breeding population nests in south Buenos Aires province and 50-70% is concentrated in the Bahía Blanca estuary (P Yorio and P. García Borboroglu in litt. 2012), an area of less than 4,500 km2 subject to growing human development (disturbance and pollution) (P Yorio and P. García Borboroglu in litt. 2012). In winter, the species disperses north, reaching Uruguay (Azpiroz in litt. 1999) and Brazil (Dias and Maurício 1998, Pacheco et al. 2009) recorded at eight localities in Rio Grande do Sul, mostly around Lagoa dos Patos (Pacheco et al. 2009), with one inland record in Córdoba (Yzurieta 1995). It was also recently sighted for the first time in Santa Catarina, thereby extending its range 380km beyond Rio Grande do Sul, its previous most northerly location (Pacheco et al. 2009). Previously considered a vagrant to Brazil, it is currently considered a regular visitor in April-August (Pacheco et al. 2009). The world population has been estimated at 4,800-7,800 pairs (P Yorio, P. García Borboroglu, and P. Petracci, unpubl. data, 2012) and 9,800-15,600 mature individuals (P Yorio and P. García Borboroglu in litt. 2012). The breeding population is variable among years, with the total number of nests estimated at 4,860 pairs in 2004, 7,790 pairs in 2007 and 5,240 in 2009 (P Yorio, P. García Borboroglu, and P. Petracci, unpubl. data) (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), and the number of mature individuals in the largest subpopulation estimated to be 7,000 mature individuals(P Yorio and P. García Borboroglu in litt. 2012). However, if non-breeding birds disperse along the coast it would seem unlikely that there are separate subpopulations with little or no mixing, thus it may be more appropriate to consider the species as having a single subpopulation. Recent information suggests that the global population data available does not allow reliable estimates of global trends (P Yorio and P. García Borboroglu in litt. 2012). The variability between years in the observed total nesting population is over 20%; highlighting the uncertainty involved and the difficulty in defining trends (P Yorio and P. García Borboroglu in litt. 2012). If there is sufficient evidence to suggest that the global population of this species should not be inferred to be in continuing decline, it would no longer qualify as Vulnerable and could warrant downlisting to Near Threatened under B1ab(ii,iii,v) of the IUCN Red List, or even Least Concern. If there is sufficient evidence to suggest that the population is in decline, this species should remain as Vulnerable under criterion B2ab(ii,iii,v), on the basis that its Area of Occupancy (AOO) is less than 2,000 km2 and it is found at ten or fewer locations (now found at six locations [(P Yorio and P. García Borboroglu in litt. 2012)]. Further information is requested on the population trends, size and distribution of this species. References: Dias, R. A. and Maurício, G. N. (1998) Lista preliminar da avifauna da extremidade sudoeste do saco de Mangueira e arresdores, Rio Grande, Rio Grande do Sul. Atualidades Ornitológicas 86: 10-11. García Borboroglu, P. and Yorio, P. (2007) Breeding habitat requirements and selection by Olrog’s Gull (Larus atlanticus), a threatened species. The Auk 124: 1201-1212. García Borboroglu, P. and Yorio, P. (2007) Comparative habitat use by syntopic Kelp Gulls (Larus dominicanus) and Olrog’s Gulls (L. atlanticus) in coastal Patagonia. Emu 107: 321-326. Pacheco, J. F., Branco, J. O. and Piacentini, V. de Q. (2009) Olrog’s Gull Larus atlanticusin Santa Catarina, Brazil: northernmost occurrence and first state record. Cotinga: 149-150. Petracci, P. F., Sotelo, M. R. and Díaz, L. I. (2008) Nuevo registro de nidificación de la Gaviota Cangrejera (Larus atlanticus) en la Reserva Natural Bahía Blanca, Bahía Falsa y Bahía Verde, Buenos Aires, Argentina. Hornero 23: 37–40. Yorio, P., Punta, G., Rabano, D., Rabuffetti, F., Herrera, G., Saravia, J. and Friedrich, P. (1997) Newly discovered breeding sites of Olrog’s Gull Larus atlanticus in Argentina. Bird Conservation International 7: 161-165. Yorio, P., Frere, E., Gandini, P. and Conway, W. (1999) Status and conservation of seabirds breeding in Argentina. Bird Conservation International 9: 299-314. Yorio, P., Bertellotti, M. and Borboroglu, P. G. (2005) Estado poblacional y de conservación de gaviotas que se reproducen en el litoral marítimo Argentino. Hornero 20(1): 53-74. Yzurieta, D. (1995) Manual de reconocimiento y evaluacion ecologica de las aves de Córdoba. Ministerio de Agricultura, Ganaderia y Recursos Renovables, Córdoba.
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