Gray Gull Leucophaeus modestus Scientific name definitions

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



Gonadal activity can be different in different years, with a peak in October–December (37). Adults copulate between August–February, and the egg-laying occurs between the end of November and mid-January (25, 19). In one colony visited over three different seasons, egg laying began at different times each season: mid- to late November, early December, and early January (21, 3). Howell et al. (3) also noted that within a colony, egg-laying was not synchronous, and extended over about a three-week period.

Nest Site


One way Gray Gull manages water loss and overheating of eggs and chicks is to choose breeding sites that have medium-sized rocks, where the small chicks look for shelter from the sun (3, 38). Despite the importance of these rocks, nests do not seem to be placed directly besides rocks (3).

Site Characteristics

The species breeds in the Atacama Desert up to 115 km from the coastline (21, 39, 20). In these areas, the air temperatures usually reach over 30°C (30), and eventually up to 40°C (FM and RS, personal observation). The reasons for this behavior are unclear, but it has been proposed that this is to avoid predation (3). The gull breeds either in aggregated colonies (with up to 20,000 breeding pairs) or in scattered isolated nests (20). The location of both the colonies and isolated nests is mobile (22, 20). Colonies can move to completely new locations after just a few years, making it hard to plan their protection; this has been attributed to years with low-food availability (22, 17).

Known colonies have an area, ranging from 0.01 km2 to 9.2 km2 (20), and the number of pairs consist of between 12–30,000 breeding pairs (20). Furthermore, there is no relationship between the distance from the coast and the colony size (20).


Construction Process

In one colony, both members of a pair would construct the nest, and was described as such: "scrapes were definitely being formed by rubbing the breast feathers in the sand and pushing posteriorly with the feet... both birds were involved in scrape-making, with each bird working on each of the scrapes at one or another time" (3). Multiple nests may be constructed within the small territory before one is selected for egg-laying (3).

Structure and Composition

The nest is a little bowl in the sand of the desert, without additional material (3). They are located in flat areas. In one colony, nests were constructed in a variety of substrates, including soft and powdery sand to loose small stones (3).


In one study, nests measured 200–230 mm in diameter and 35 mm deep (3).



The pores in the eggshells are smaller than expected for this size of gull, and this is understood as an adaptation to avoid the loss of water in the middle of the most arid desert in the world (40). As a handicap, this adaptation reduces the oxygen exchange with the environment, which produces a protracted incubation period (41).


Egg length, 58.3 ± 0.57 mm (n = 14; 21); egg width, 41.3 ± 0.28 mm (n = 14; 21).


Mean egg mass in one colony was 51.8 g (range 42–62 g, n = 89 eggs). In clutches with two eggs, one egg was almost always larger than the other, and the mean difference in mass was 5.2 g (n = 23 clutches) (3).

Color and Surface Texture

Eggs have been described as ivory-colored with reddish spots, especially near the base; in one study, eggs were described as sometimes approaching pale ochraceous salmon in base color (42). Some eggs have been described as being very faintly spotted (3).

Clutch Size

In one colony, most nests had 1 egg (39.2% of the nests) or 2 eggs (59.8% of the nests), but a small number can have 3 or 4 eggs (20). In another colony, 52% of nests had 1 egg, 47% had 2 eggs, and 1% had 3 eggs (n = 182 nests) (3).


Onset of Broodiness and Incubation in Relation to Laying

Incubation begins with the laying of the first egg (3).

Incubation Period

Incubation usually takes 29–30 d (43), but can take up to 32 d (3).

Parental Behavior

During incubation, one of the parents stays at the nest during the day to shade the eggs to avoiding overheating (36). The switch of incubation duties between parents happens during the night (3, 43). In a study of one colony, adults sat on eggs during the night, and would stand to shade the eggs by midday or early afternoon; during windy events, birds would sit on their eggs even during the day, as the wind would cool the eggs significantly (3).

Hardiness of Eggs Against Temperature Stress: Effect of Egg Neglect

Attended eggs were maintained at a temperature between 33–38°C (3). Eggs that were left unattended and exposed to the sun during the day reached internal temperatures above 44°C, which has been shown to be lethal for chicken eggs (3).


Preliminary Events and Vocalizations

In one study, eggs hatched 2–3 d after the first pip appeared (3).

Shell Breaking and Emergence

Howell et al. (3) found that in two egg clutches, hatching was not synchronous, and that the second chick would hatch 1–2 d after the first chick hatched.

Young Birds

Growth and Development

Chicks are pale gray (occasionally buffy), with obscure black spots on sides of their head, throat, back, and flanks. In one colony, at hatching, chicks had a mean mass of 34 g (range of 31–38 g, n = 8; 3). The chick's growth takes 60–70 d before fledging (44). In one study, Guerra et al. (44) found that tarsus length increased fastest, and reached an asymptote in growth rate after 50 d, while culmen length increased at a steady rate until it reached an asymptote at 100 d.

Parental Care


After hatching, the parents stay on the nest during the day for the first ten days (44). When at the nest, adults provide shade to chicks to keep them from overheating (3). At one colony, adults were seen to attack chicks, including their own, that were close to but outside of the nest; chicks would respond by performing a submissive display, where they would crouch and point its bill down, after which point adults would usually stop attacking. Once a chick reached the nest and was under an adult, the adult would brood or provide shade to the chick. Any chick that reaches a nest under an adult would be brooded, not just the parent's chick (3).


Chicks are primarily fed Peruvian anchovy (Engraulis ringens) and Pacific mole crab (Emerita analoga) (44). Parents return to the nest at night to feed chicks. One adult returning to the colony was carrying about 40 g of food in the lower esophagus and proventriculus. In one study, adults were found to regurgitate at least some food immediately upon returning to the nest site, and may regurgitate food over the course of the entire night, sometimes even into the morning. Chicks would either eat food from the ground, or eat food that was passed directly to them by the adult (3).

Cooperative Breeding

Does not exhibit cooperative breeding.

Brood Parasitism by Other Species

Its nests are not parasitized by other birds.

Fledgling Stage

When fledglings leave the nests, they fly on their own to the sea.

Immature Stage

Information needed.

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