SPECIES

Gray Gull Leucophaeus modestus Scientific name definitions

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

Behavior

Locomotion

Flight

When flying, it can gain altitude using thermal wind currents (21).

Walking

Walks with slight side-to-side body movements (RS and FM, personal observation).

Self-Maintenance

In the desert, the adults pant to avoid losing heat through evaporation. They hold their bodies to lose heat by convection (36, 25). Apparently, birds take baths mainly in fresh water (FM and RS, personal observation).

Agonistic Behavior

Communicative and Physical Interactions

In one study of agonistic behavior at a coastal location in Chile, Moynihan (35) noted physical attacks were very rare. Many different physical agonistic displays are given, which Moynihan (35) describes as follows: (1) Upright Posture (including "Aggressive" and "Anxiety" Upright Postures, which are extreme variations of typical Upright Posture), during which the neck is stretched upward and the head and bill is held horizontally or pointed slightly downward (usually during "Aggressive" variation), the wings held slightly away from the body, the plumage is sleek and held close to the body, and the body is held horizontally or with the breast directed somewhat upward; (2) Head-flagging, during which a bird turns its head horizontally before returning to face another bird; (3) Swinging, which is similar to Head-flagging, involves multiple turns of the head from side-to-side; (4) Gaping, in which a bird opens its bill wide and exposes the bright red-orange mouth lining. During these various displays, birds may also adjust the feathers on their head, either raising them or smoothing them, which gives the head a different shape and which communicates different intentions: in the "Crested" display, the feathers on the back of the crown are raised while all others are smoothed down; in the "Egg-shaped" display, the feathers on the nape and back of the head are raised, giving the head an egg shape; and in the "Angular" display, all the feathers on the head are smoothed very flat, giving the head an angular and "bony" appearance (35). In addition to these ritualized postures, Gray Gull also has a series of ritualized postures associated with vocalizations, including the Oblique Posture, Low Oblique Posture, and Head-tossing, which are associated with the Long Call (see Vocalizations), an extreme Low Oblique Posture associated with the Mew Call, and the Choking Display (35). During the Oblique Posture, the neck is stretched out and slightly arched upward, and the head held high, with the bill either pointing upward at an angle or straight up, while the wings are held slightly away from the body and the neck feathers are slightly ruffled; the Low Oblique Posture typically involves arching the neck but holding the head horizontal or slightly pointed downward; Head-tossing, which can be used in conjunction with either Oblique Posture during the Long Call, involves throwing the head back so that nearly touches the back, then bringing the head back forward (35). Head-tossing after the Long Call is often associated with agonistic behavior, while Head-tossing in conjunction with other displays is more frequent during Courtship (see Sexual Behavior). The Choking Display involves a bird stretching its neck out and forward, and pointing the bill almost straight down. These various postures can also be paired with Head-flagging and adjustments of the feathers on the head (35). In addition to these, which are given by adults and immatures, immature birds will also use Hunched and Forward Hunched postures, which involve ruffling and raising of the scapular and back feathers and holding the head forward at and upward angle (35).

The various ritualized behaviors described are used in a variety of circumstances, and reflect different levels of aggression, and can also be combined in a variety of ways (35). The Upright Posture, for example, is relatively common and represents a low aggression level, though aggression could escalate to chase and physical attack. For example, the "Aggressive" Upright Posture is often given right before a bird attacks, while the "Anxiety" Upright Posture is often given by a retreating bird. Head-flagging and Head-swinging were often used in combination with other postures, and may be used to emphasize the signal of the base behavior. Gaping was always noted directly before a bird attacked another bird (35). Long Calls, and their associated physical displays, appear to be used in more prolonged interactions, and can also be directed toward opponents up to about 10 m away (35). Head-tossing by immature birds can be associated with disputes over food or resting places (35). Many of the same displays are also used during Courtship (see Sexual Behavior).

Pursuit flights were relatively rare in one study of agonistic behavior. Moynihan (35) noted that during pursuit flights, individuals would perform Swoop and Soar displays, both of which were described as "shallow." During these flights, birds sometimes also gave a Plaintive Charge Call, a series of drawn-out Contact Vocalizations (see Vocalizations).

In addition to the various ritualized behaviors described, several unritualized agonistic behaviors have been noted (35). However, Moynihan (35) noted that aggressive ritualized displays are more common, and seem to prevent attacks by other gulls.

Territorial Behavior

On feeding grounds along the coast at the beginning of the breeding season (November), individuals and pairs appeared to defend small territories, and would chase or react with an Upright or Low Oblique posture; these reactions usually occurred only when pairs were engaged in courtship displays, including courtship feeding (35, 3). At one breeding colony, territorial behavior was most pronounced during the pre-egg period. Howell et al. (3) observed one bird that stood on rocks in the middle of its territory, where it gave Long Calls. When other birds approached the territory boundary (see Population Spatial Metrics), the territory holder would respond with an Upright Posture or Long Call, or move toward the intruder using the Low Oblique Posture. Territorial behavior appeared to decline and disappear once adults were incubating, with individuals just pecking at birds that got too close to the nest (3).

Sexual Behavior

Mating System and Operational Sex Ratio

Presumably monogamous based on patterns of courtship and parental care, but not specifically studied.

Courtship, Copulation, and Pair Bond

Unmated males and females perform a series of displays at coastal sites to find potential mates (though females appear to do this less frequently). These displays at the early stages of pair formation are described in detail by Moynihan (35), and the following summary is based on that description, unless otherwise noted. Unmated birds, mostly males, defended a small patch of coast that was mostly flat (see Population Spatial Metrics), where they would perform "invitation" displays, which involved performing the Long Call toward birds that flew over, as well as running among other gulls in the Low Oblique Posture. In Moynihan's (35) observations, running through a group of birds using the Low Oblique Posture was the most common "invitation" display, though some individuals spent equal amounts of time running versus performing Long Calls towards flyover birds from their small territory.

Once a bird attracted a potential mate, the two would perform a series of ritualized displays that served to establish a pair bond. The first stage in establishing a pair bond was the "greeting ceremony," which involves three stages: 1) both members of the pair would give Long Calls, starting with the female, and with the male responding while in the Oblique Posture; 2) both members of the pair may give the Mew Call while in the Low Oblique Posture, often with Head-swinging and Head-flagging movements; and 3) both members of the pair may adopt the Upright Posture, often with Head-flagging and Head-swinging, as well as rapid preening movements of the back and/or wings; one or both members of the pair may also "Point" during the Upright Posture during which time a bird turns its head so the bill points directly at the other bird for a sustained period of time. When in the Upright Posture, the pair may walk around together. Following these displays, the male, or both the male and female, sometimes take flight and give Long Call notes before landing again; the flight display is relatively rare (35).

During later stages of pairing, a further series of displays between the male and female are given, which are described in detail by Moynihan (35) and Howell et al. (3), and summarized here. In the first stages of the Courtship display, one bird, usually the female, approaches the other in the Hunched Posture and paces back and forth, sometimes also adopting the Low Oblique Posture, while also giving the Mew Call; the other member of the pair may also give the Mew Call in response. Following this, both members of the pair perform the Head-tossing display, while the bill is slightly open and giving a soft Kioo call. Following this, the female "repeatedly nibbles at the male's bill," to which the male responds by regurgitating fish, which the female then eats; the male regurgitates fish while in the Low Oblique Posture (3). After the female ate the fish provided by the male, both members of the pair would rinse their bills in water, dipping their bill into water and shaking their head from side-to-side (3). After this courtship feeding, a male will then walk around the female in a Hunched or Low Oblique posture; if the female stands still and crouches, males will mount and copulate with the female. A male may balance on the back of the female for up 90–180s, and attempt copulation several times during this period (3); copulation takes from 3–6 s (Pantoja-Maggi, eBird).

Extra-Pair Mating Behavior

Howell et al. (3) noted instances when a second male would interrupt the first male during the copulation process and proceed to attempt to mate with the female. It is unknown whether these mating attempts were successful, or whether they produced extra-pair young.

Social and Interspecific Behavior

Degree of Sociality

When foraging and roosting, either in the sea or on the beach, flocks can be comprised of thousands of individuals.

Nonpredatory Interspecific Interactions

It can form aggregations with other species of gulls (mainly Franklin's Gull (Leucophaeus pipixcan) and Kelp Gull (Larus dominicanus)) and terns (especially South American Tern (Sterna hirundinacea) and Elegant Tern (Thalasseus elegans)).

Predation

The main predators at the nests include Turkey Vulture (Cathartes aura), Peregrine Falcon (Falco peregrinus), and the Culpeo fox (Lycalopex culpaeus) (30, 20). Dogs will also prey on the chicks at coastal colonies (20).

Anecdotally, a Gray Gull has been recorded being eaten by a Peruvian Pelican (Pelecanus thagus) at sea (J. P. Salgado, personal communication).

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