Demography and Populations

More study is needed. In some anecdotal observations from one colony, Howell et al. (3 Howell, T. R., B. Araya, and W. R. Millie (1974). Breeding biology of the Gray Gull, Larus modestus. University of California Publications in Zoology 104:1-57. Close ) noted many dead chicks of all ages, and suggested that chicks may have died as a result of exposure, starvation, or a combination of the two.

Nothing known.

Little known. The acanthocephalan (Profilicollis altmani) can parasitize its intestines (45 Gonzales-Viera, O., C. Luján-Vega, A. Chavera–Castillo, J. Cárdenas-Callirgos, and V. M. Tantaleán (2009). Lesiones patológicas causadas por Profilicollis altmani (Perry, 1942) Van Cleave, 1947 (Acanthocephala) en una gaviota gris (Leucophaeus modestus) (Tschudi, 1843) de la Costa Peruana. Neotropical Helminthology 3(2):115-120. Close ).

Causes of mortality for Gray Gull include collisions with man-made structures (25 Aguilar R., M. Perucci, T. Cisternas, M. Torres, M. Silva, A. Marín, A. Silva, and P. Bolados (2013). La nidificación de la Gaviota Garuma y su vulnerabilidad a las actividades antrópicas en el Desierto de Atacama. CULTAM. Technical report. Close , 30 Malinarich, V. (2016). Diagnóstico poblacional de la gaviota garuma Leucophaeus modestus (Tschudi, 1843), en la Zona Norte de Chile. Unidad de Recursos Naturales Renovables. Servicio Agrícola y Ganadero (SAG), Región de Tarapacá Report. 39 pp. Close , 20 Aguilar-Pulido R., C. Catoni, G. Luna-Jorquera, M. Perucci, G. Dell'Omo, C. Zavalaga, and A. Simeone (2021). Distribución, características y situación actual de las colonias reproductivas de la Gaviota garuma (Leucophaeus modestus) en el desierto de Atacama, Norte de Chile. Revista Chilena de Ornitología 27:21-36 Close ) and predation. During years with the El Niño Southern Oscillation (ENSO), large numbers of juveniles have been recorded starving and desperately looking for shade near roads or other human infrastructure. Presumably, this means they have been abandoned by adults due to lack of food. This may represent an important cause of mortality in some years (46 Guerra, C. G., L. C. Fitzpatrick, R. E. Aguilar, and B. J. Venables (1988). Reproductive consequences of El Niño-Southern Oscillation in Gray Gulls (Larus modestus). Colonial Waterbirds 11(2):170-175. Close ).

Individual Distance

Outside of breeding colonies when along the coast, Howell et al. (3 Howell, T. R., B. Araya, and W. R. Millie (1974). Breeding biology of the Gray Gull, Larus modestus. University of California Publications in Zoology 104:1-57. Close ) found that individuals were 0.8–2.4 m apart when in large, dense flocks.

Territory Size

Known colonies have an area of between 0.01 km² and 9.2 km² (20 Aguilar-Pulido R., C. Catoni, G. Luna-Jorquera, M. Perucci, G. Dell'Omo, C. Zavalaga, and A. Simeone (2021). Distribución, características y situación actual de las colonias reproductivas de la Gaviota garuma (Leucophaeus modestus) en el desierto de Atacama, Norte de Chile. Revista Chilena de Ornitología 27:21-36 Close ). In one study of a colony from the Pampa del Miraje, Howell et al. (3 Howell, T. R., B. Araya, and W. R. Millie (1974). Breeding biology of the Gray Gull, Larus modestus. University of California Publications in Zoology 104:1-57. Close ) estimated one territory in the pre-egg stage they studied to be about 10 m by 3 m. After eggs were laid however, territories were not as strongly defended, and nests could be placed as close as 1–2 m from another nest (3 Howell, T. R., B. Araya, and W. R. Millie (1974). Breeding biology of the Gray Gull, Larus modestus. University of California Publications in Zoology 104:1-57. Close ). In one colony, in a 100 x 100 m section, 110 nests were found; nests were not distributed evenly, and in some smaller grids of 10 x 10 m, between 0 and 4 nests were counted (3 Howell, T. R., B. Araya, and W. R. Millie (1974). Breeding biology of the Gray Gull, Larus modestus. University of California Publications in Zoology 104:1-57. Close ). In one study, the temporary territories defended by unmated birds along the coast away from breeding colonies as they displayed to attract a mate varied in size between about 0.8–2.3 m² [9–25 ft²] (35 Moynihan, M. M. (1962). Hostile and sexual behaviour patterns of South American and Pacific Laridae. Behaviour. Supplement 8:1-365 Close ).

Numbers

Its population size has been estimated at 25,000 individuals (47 Delany, S., and D. Scott. (2006). Waterbird population estimates. 4th edition. Wetlands International, Wageningen, The Netherlands Close ), but this is an underestimate. In Chile alone, on Arica's beaches, there are estimations of up to 20,000 individuals; in Río Seco there are estimates of 10,000 individuals; and near the Loa River, there are estimates of up to 8,000 individuals (19 Medrano, F. (2018). Gaviota garuma. In Atlas de las Aves Nidificantes de Chile (F. Medrano, R. Barros, H. Norambuena, R. Matus, and F. Schmitt, Editors), Red de Observadores de Aves y Vida Silvestre de Chile, Santiago, Chile. pp. 234-235. Close ). Also, only in Cerro Chanchito were 30,000 breeding pairs (60,000 mature individuals; 20 Aguilar-Pulido R., C. Catoni, G. Luna-Jorquera, M. Perucci, G. Dell'Omo, C. Zavalaga, and A. Simeone (2021). Distribución, características y situación actual de las colonias reproductivas de la Gaviota garuma (Leucophaeus modestus) en el desierto de Atacama, Norte de Chile. Revista Chilena de Ornitología 27:21-36 Close ). In Peru, the population size is also unknown. Still, there are records of up to 25,000 individuals in the Paracas National Reserve (Roberson and Singer, eBird) and up to 7,200 individuals in Humedales Punta Bombon (Seeholzer, eBird). Consequently, presumably, the population size would exceed 80,000 individuals.

Trends

The population status has been classified as decreasing (48 BirdLife International (2022). Species factsheet: Larus modestus. Downloaded from http://www.birdlife.org on 26/07/2022 Close ).

Nothing is known, but apparently Gray Gull is highly sensitive to food shortages during oceanographic events like the El Niño Southern Oscillation (20 Aguilar-Pulido R., C. Catoni, G. Luna-Jorquera, M. Perucci, G. Dell'Omo, C. Zavalaga, and A. Simeone (2021). Distribución, características y situación actual de las colonias reproductivas de la Gaviota garuma (Leucophaeus modestus) en el desierto de Atacama, Norte de Chile. Revista Chilena de Ornitología 27:21-36 Close ).