Gray Gull Leucophaeus modestus Scientific name definitions

Fernando Medrano, Ignacio Escobar Gutiérrez, and Rodrigo Silva
Version: 2.0 — Published December 23, 2022

Demography and Populations

Measures of Breeding Activity

More study is needed. In some anecdotal observations from one colony, Howell et al. (3) 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.

Life Span and Survivorship

Nothing known.

Disease and Body Parasites

Little known. The acanthocephalan (Profilicollis altmani) can parasitize its intestines (45).

Causes of Mortality

Causes of mortality for Gray Gull include collisions with man-made structures (25, 30, 20) 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).

Population Spatial Metrics

Individual Distance

Outside of breeding colonies when along the coast, Howell et al. (3) 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). In one study of a colony from the Pampa del Miraje, Howell et al. (3) 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). 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). 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).

Population Status


Its population size has been estimated at 25,000 individuals (47), 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). Also, only in Cerro Chanchito were 30,000 breeding pairs (60,000 mature individuals; 20). 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.


The population status has been classified as decreasing (48).

Population Regulation

Nothing is known, but apparently Gray Gull is highly sensitive to food shortages during oceanographic events like the El Niño Southern Oscillation (20).

Recommended Citation

Medrano, F., I. Escobar Gutiérrez, and R. Silva (2022). Gray Gull (Leucophaeus modestus), version 2.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.grygul.02