Larus ridibundus
black-headed gull
(Also: common black-headed gull)

The breeding range of Larus ridibundus is very broad, extending from the southern tip of Greenland and all of Iceland down through the majority of Europe and Central Asia. The Kamchatka Peninsula forms this species' eastern range; Ussuriland in Russia and Heilongjiang in northeast China lie at the extreme southeastern part of this range. Larus ridibundus is uncommon in northeastern North America.

The northern population of this species is migratory. Birds residing at lower latitudes, however, tend to be non-migratory. Most birds in the western Palearctic breed in central and north European wetlands and migrate to their winter grounds in the Mediterranean basin. Birds that breed in Scandinavia migrate to the Brittish Isles, and the majority migrate farther, flying along the Atlantic Coast to West Africa. Birds from Central Asia migrate south to India, Malaysia, and Philippines. North Africa, particularly Egypt, is a common destination for L. ridibundus (Howard and Moore, 1991; Cantos et al., 1994). (Cantos, et al., 1994; Howard and Moore, 1991)

Larus ridibundus inhabits the temperate zone to the rim of the Palearctic boreal forests. It is found mostly at low attitudes and in or around placid, shallow coastal or inland water bodies, including rivers and their estuaries. In some areas, such as Scandinavia, it has adapted to settle in salt marshes, clay pits, and coastal dunes and offshore islands. The distribution of this highly adaptable species has increased to encompass areas near canals or sewage treatment facilities (Howard and Moore, 1991; Cantos et al., 1994). (Cantos, et al., 1994; Howard and Moore, 1991)

In accordance with its common name, L. ridibundus is distinguished by its dark brown or grayish-black frontal hood. Its eye crescents (primarily behind the eye), neck, and underparts are all white as is the tail. The upper wing coverts, secondaries, inner primaries, and back are gray. The secondaries are tipped with white; the white outer primaries have black tips and edges. Other identifying characteristics of L. ridibundus include its red legs and bill, and dark brown eyes. Non-breeding adults have a white head, with only some blackish coloring on their nape. Juvenile birds are recognized by the beige to darker brown markings on their back and upper wing coverts. Also, they have a black terminal tail band. The species is sexually monomorphic. Larus ridibundus ranges from 37 to 43 cm in length and has a wingspan from 94 to 110 cm (Howard and Moore, 1991). (Howard and Moore, 1991)

Information on the mating system of these animals is not available.

After returning to its colonies between late February and late March, L. ridibundus lays its eggs in late April and May. Females lay from one to three eggs and incubate them for 22 to 26 days. Larus ridibundus is strongly inclined to nest near vegetation. At the very beginning of the breeding season, male L. ridibundus congregate in larges areas close to the nesting colony before females arrive. In these areas, termed "clubs," each bird is semi-territorial in that it does not stay in one particular area for a considerable amount of time. A male first regards visiting females as a threat, showing his aggressive oblique display and making long calls. In response, the female extends her neck upward and faces away, a display that makes her sex and potential as a mate known. The male then mitigates his response. The mating process continues when females keep coming back to a selected male, staying with him for progressively longer periods of time. The potential pair exchanges mutual displays. Their courtship culminates in the male regurgitating food for the female, an act followed by copulation.

As soon the pair of L. ridibundus settle into their colony area, they become territorial and defend their area against intruders (primarily conspecifics). They clearly mark their pairing territories, which range in size from 9 to 11 square meters. Their territorial boundaries are subject to slight changes as dictated by daily disputes with neighbors or intruders.

Colonies of L. ridibundus are made up of 11 to 100 breeding pairs; however, a few colonies reach numbers greater than 10,000 birds. These colonies are loosely divided: a breeding bird will eventually become accustomed to its neighbors, but will become very aggressive towards birds that it cannot recognize.

Research has shown that L. ridibundus is a philopatric species in which individuals are inclined to return to breed in the subcolony in which they were born. This tendency is especially evident within a large colony located in a relatively unstable habitat. Philopatry encourages the establishment of kin groups and eventually heightened cooperation among neighbors (Howard and Moore, 1991; Moynihan, 1955; Prevot-Julliard et al., 1998; Gill, 1995). (Gill, 1995; Howard and Moore, 1991; Moynihan, 1955; Prevot-Julliard, et al., 1998)

Three displays signaling agonistic behavior specific to L. ridibundus have been identified in its young: the oblique, in which an erect posture complemented by a series of loud and strident calls that decrease in length; the forward, in which the neck is extended, the head is in front of the body, and the bill is horizontal to the ground; and choking, in which the bird leans toward the ground, points its bill downward and proceeds to emit a soft call of short notes in rapid succession.

According to one study, at least six variables determined the outcome of a fight in L. ridibundus: ownership (i.e. who initiated the fight), semiterritoriality, age, body mass, food availability, and location. Scientists propose that a bird might learn to shift from obvious aggression to the aforementioned displays when it realizes that the latter are just as effective in warding off opponents. This finding may help explain why immature L. ridibundus that have not yet learned the art of display instigate and win more fights than adults, and also take more risks in interactions with other species. It has been hypothesized that fighting can be beneficial to immature birds in that it helps them to gain "self-knowledge," (i.e. become aware of their own strengths).

In order to protect itself from predators, L. ridibundus displays both aggressive and fleeing behaviors. The type of behavior depends largely on the specific predator. For instance, predators that primarily take chicks and eggs, such as crows and larger gulls, warrant aggressive behavior. Attacks on adult birds by peregrine falcons (Falco peregrinus), however, cause dense flocks to fly away. Threats to both adults and the brood, such as humans and foxes, precipitate both aggressive and fleeing behavior. Factors like colonial nesting and breeding synchronization help to keep predators at bay (Groothuis and Van Mulekom, 1991; Nuyts et al., 1996; Kruuk, 1964). (Groothuis and Van Mulekom, 1991; Kruuk, 1964; Nuyets, et al., 1996)

Aquatic and terrestrial insects, earthworms, and marine invertebrates compose the bulk of L. ridibundus diet. This species also feeds on fish and grains, although to a lesser extent. Studies have shown that adults store greater nutrient reserves (fat and protein depots) not only for migration, but for reproductive activity as well.

This species forages by swimming and snatching food from the water surface, or by submerging its head under the water surface. These birds forage along coastal areas. Adults have more efficient foraging skills than do immature birds. Due to this inefficiency, immature birds feed in areas separate from the adult sites. Immature birds also display greater aggression, often winning fights over food. Interestingly, they are also more brazen in approaching humans, thus gaining a better chance to secure food.

The gulls are also kleptoparasitic on occasion, meaning that they steal food that has already been caught by a member of the same or different species. For example, in the Netherlands, sandwich terns (Sterna sandvicensis) nearly always stake their breeding ground in close proximity to colonies of L. ridibundus. In this arrangement, the gulls help protect the terns by driving out both avian and ground predators. The gulls, though, are major predators of tern eggs and chicks, and steal fish that tern parents have procured for their chicks. Larus ridibundus prefers to eat tern chicks up to two weeks old. Several scientists have suggested that kleptoparasitism occurs more often when other food sources are scarce.

Larus ridibundus displays high flexibility in diet. In western Europe, for instance, it has come to rely on human trash as an artificial food source. Researchers have attributed the significant increase in distribution of L. ridibundus to this influx of readily available food (Howard and Moore, 1991; Cantos et al., 1994; Stienen and Brenninkmeijer, 1999; Nuyts et al., 1996). (Cantos, et al., 1994; Howard and Moore, 1991; Stienen and Brenninkmeijer, 1999)

Larus ridibundus is not globally threatened, with a population approximated at two million pairs or perhaps more. As many as 1,500,000 to 1,800,000 have been estimated in Western Europe alone. From 1950 to 1980, this species underwent an impressive increase in numbers and subsequent increase in distribution (Howard and Moore, 1991). (Howard and Moore, 1991)

Although this is a Palearctic species, the numbers of L. ridibundus in eastern North America have risen from the 1950s; it first bred in eastern Canada in 1977. Today there are four known breeding sites of L. ridibundus in Canada and New England, but fewer than 20 breeding pairs (Howard and Moore, 1991). (Howard and Moore, 1991)