Gull

Gulls range in size from the little gull, at 120 grams (4+1⁄4 ounces) and 29 centimetres (11+1⁄2 inches), to the great black-backed gull, at 1.75 kg (3 lb 14 oz) and 76 cm (30 in). They are generally uniform in shape, with heavy bodies, long wing, and moderately long necks. The tails of all but three species are rounded; the exceptions being Sabine's gull and swallow-tailed gulls, which have forked tails, and Ross's gull, which has a wedge-shaped tail. Gulls have moderately long legs, especially when compared to the similar terns, with fully webbed feet. The bill is generally heavy and slightly hooked, with the larger species having stouter bills than the smaller species. The bill colour is often yellow with a red spot for the larger white-headed species and red, dark red or black in the smaller species.^[13]

Gulls are a generalist species that can thrive in various environments and survive on a widely varied diet. They are the least specialised of all the seabirds, and their morphology allows for equal adeptness in swimming, flying, and walking. They are more adept walking on land than most other seabirds, and the smaller gulls tend to be more manoeuvrable while walking. The walking gait of gulls includes a slight side to side motion, something that can be exaggerated in breeding displays. In the air, they are able to hover and they are also able to take off quickly with little space.^[13]

The general pattern of plumage in adult gulls is a white body with a darker mantle; the extent to which the mantle is darker varies from pale grey to black. A few species vary in this, the ivory gull is entirely white, and some like the lava gull and Heermann's gull have partly or entirely grey bodies. The wingtips of most species are black, which improves their resistance to wear and tear, usually with a diagnostic pattern of white markings. The head of a gull may be covered by a dark hood or be entirely white. The plumage of the head varies by breeding season; in nonbreeding dark-hooded gulls, the hood is lost, sometimes leaving a single spot behind the eye, and in white-headed gulls, nonbreeding heads may have streaking.^[13]

Gulls have a worldwide cosmopolitan distribution. They breed on every continent, including the margins of Antarctica, and are even found in the high Arctic. They are less common in the tropics, although a few species do live on tropical islands such as the Galapagos and New Caledonia. Many species breed in coastal colonies, with a preference for islands; one particular species, the grey gull, breeds in the interior of dry deserts far from water. Considerable variety exists in the Laridae family, and species may breed and feed in marine, freshwater, or terrestrial habitats.^[13]

Most gull species are migratory, with birds moving to warmer habitats during the winter, but the extent to which they migrate varies by species. Some migrate long distances, notably Sabine's gull, which migrates from the Arctic coasts to wintering grounds off the west coasts of South America and southern Africa, and Franklin's gull, which migrates from Canada to winter off the west coast of South America. Other species move much shorter distances and may simply disperse along the coasts near their breeding sites.^[13]

A big influence on non-breeding gull distribution is the availability of food patches. Human fisheries especially have an impact, since they often provide an abundant and predictable food resource.^[14] Two species of gulls dependent on human fisheries are Audouin's gull (Ichthyaetus audouinii) and lesser black-backed gulls (Larus fuscus); their breeding distributions (especially the black-backed gull) are heavily impacted by human fishing discards and fishing ports.^[14]

Other environmental drivers that structure bird habitat and distribution are human activity and climate impacts. For example, waterbird distribution in Mediterranean wetlands is influenced by changes in salinity, water depth, water body isolation and hydroperiod, all of which have been observed to affect the bird community structure in both a species- and guild-specific way.^[15] Gulls in particular have high associations with salinity levels, which were found to be the main environmental predictor for waterbird assemblage.^[15]

Charadriiform birds drink salt water, as well as fresh water, as they possess exocrine glands located in supraorbital grooves of the skull by which salt can be excreted through the nostrils to assist the kidneys in maintaining electrolyte balance.^[16] Gulls are highly adaptable feeders that take a wide range of prey opportunistically. The food taken by gulls includes fish, and marine and freshwater invertebrates, both alive and already dead; terrestrial arthropods and invertebrates such as insects and earthworms; rodents, eggs, carrion, offal, reptiles, amphibians, seeds, fruit, human refuse, and even other birds. No gull species is a single-prey specialist, and no gull species forages using only a single method. The type of food depends on circumstances; terrestrial prey, e.g. seeds, fruit and earthworms, is more common during the breeding season, while marine prey is more common in the nonbreeding season when birds spend more time on large bodies of water.^[13]

Gulls not only take a wide range of prey, they also display great versatility in how they obtain it; prey can be caught in the air, on water, or on land. A number of hooded species are able to hawk insects on the wing, although the larger species perform this feat more rarely. Gulls on the wing snatch items both off the water and off the ground, and they are able to plunge-dive into water to catch prey. Smaller species are more manoeuvrable and better able to hover-dip fish from the air. Dipping is common when birds are sitting on the water, and gulls may swim in tight circles or foot paddle to bring marine invertebrates up to the surface.

Food is also obtained by searching the ground, often on the shore among sand, mud or rocks. Larger gulls tend to do more feeding in this way. Gulls may also engage in foot paddling in shallow water for invertebrates^[17] or on wet grass for earthworms.^[18] One method of obtaining prey involves dropping heavy shells of clams and mussels onto hard surfaces.^[13] Gulls may fly some distance to find a suitable surface on which to drop shells, and there is evidently a learned component to the task because older birds are more successful than younger birds.^[19] While overall feeding success is a function of age, the diversity in both prey and feeding methods is not. The time taken to learn foraging skills may explain the delayed maturation in gulls.^[13]

Gulls have only a limited ability to dive below the water surface to feed on deeper prey. To obtain prey from a greater depth, many species of gulls feed in association with other animals, where marine hunters drive prey to the surface when hunting.^[13] Examples of such associations include four species of gulls that feed around plumes of mud brought to the surface by feeding grey whales,^[20] and also between orcas (the largest dolphin species) and kelp gulls (among other seabirds).^[21]

Looking at the effect of humans on gull diet, overfishing of target prey such as sardines have caused a shift in diet and behaviour. Analysis of the yellow-legged gull's (Larus michahellis) pellets off the northwest coast of Spain has revealed a shift from a sardine to crustacean-based diet.^[22] This shift was linked to higher fishing efficiency and thus overall fish stock depletion.^[22] Lastly, closure of nearby open-air landfills limited food availability for the gulls, further creating a stress on their shift in diet.^[22] From 1974 to 1994, yellow-legged gull populations on Berlenga Island, Portugal, increased from 2600 to 44,698 individuals. Analyzing both adult and chick remains, researchers found a mixture of both natural prey and human refuse. The gulls relied substantially on the Henslow's swimming crab (Polybius henslowii). Yet, in times when local prey availability is low, the gulls shift to human-related food. These temporal shifts from marine to terrestrial prey highlight the resilience of adult gulls and their ability to keep chick condition consistent.^[23] Human disturbance has also been shown to have an effect on gull breeding, in which hatching failure is directly proportional to the amount of disturbance in a given plot.^[24] Certain gull breeds have been known to feast on the eyeballs of baby seals and directly pilfer milk from the elephant seal's teat.^[25][26]

Gulls are monogamous and colonial breeders that display mate fidelity which normally lasts for the life of the pair. Divorce of mated pairs does occur, but it apparently has a social cost that persists for a number of years after the break-up. Gulls also display high levels of site fidelity, returning to the same colony after breeding there once and even usually breeding at the same location within that colony. Gull colonies can vary from just a few pairs to over a hundred thousand pairs, and may be exclusive to that gull species or shared with other seabird species. A few species nest singly, and single pairs of band-tailed gulls may breed in colonies of other bird species. Within colonies, gull pairs are territorial, defending an area of varying size around the nesting site from others of their species. This area can be as large as a 5-metre radius around the nest in the European herring gull to just a tiny area of cliff ledge in the kittiwakes.^[13]

Most gulls breed once a year and have predictable breeding seasons lasting for three to five months. Gulls begin to assemble around the colony for a few weeks prior to occupying it. Existing pairs re-establish their pair-bonds, and unpaired birds begin courting. Pairs then move back into their territories, and new males establish new territories and attempt to court females. Gulls defend their territories from rivals of both sexes using calls and aerial attacks.^[13]

Nest building is an important part of the pair-bonding process. Most gull nests are mats of herbaceous matter with a central nest cup. Nests are usually built on the ground, but a few species establish their nests on cliffs (the usual preference for kittiwakes), and some choose to nest in trees and high places (e.g. Bonaparte's gulls). Species that nest in marshes need to construct a nesting platform to keep the nest dry, particularly species that nest in tidal marshes. Both sexes gather nesting material and build the nest, but the division of labour is not always exactly equal.^[13] In coastal towns, many gulls nest on rooftops and can be observed by nearby human residents.

Clutch size is typically three eggs, although some of the smaller gulls only lay two, and the swallow-tailed gull produces a single egg. Birds synchronise their laying within colonies, with a higher level of synchronisation in larger colonies. The eggs of gulls are usually dark tan to brown or dark olive with dark splotches and scrawl markings, and they are well camouflaged. Both sexes incubate the eggs; incubation bouts last between one and four hours during the day, and one parent incubates through the night.^[13] Research on various bird species, including gulls, suggests that females form pair bonds with other females to obtain alloparental care for their dependent offspring, a behaviour seen in other animal species, such as elephants, wolves, and the fathead minnow.^[27]

Lasting between 22 and 26 days, incubation begins after the first egg is laid but is not continuous until after the second egg is laid, meaning that the first two chicks hatch at about the same time, and the third some time later. Young chicks are brooded by their parents for about one or two weeks, and often at least one parent stays behind to guard the chicks until they fledge. Although the chicks are fed by both parents, early on in the rearing period the male does most of the feeding and the female most of the brooding and guarding.^[13]

The family Laridae was introduced (as Laridia) by the French polymath Constantine Samuel Rafinesque in 1815.^[28][29] The taxonomy of gulls is confused by their widespread distribution zones of hybridisation leading to gene flow. Some have traditionally been considered ring species, but research has suggested that this assumption is questionable.^[30] Before the 21st century, most gulls were placed in the genus Larus, but this arrangement is now known to be polyphyletic, leading to the resurrection of the genera Ichthyaetus, Chroicocephalus, Leucophaeus, Saundersilarus, and Hydrocoloeus.^[1] Some English names refer to species complexes within the group:

• Large white-headed gull is used to describe the 18 or so herring gull-like species, from California gull to lesser black-backed gull in the taxonomic list below.
• White-winged gull is used to describe the four pale-winged, high Arctic-breeding taxa within the former group; these are Iceland gull, glaucous gull, Thayer's gull, and Kumlien's gull.

In common usage, members of various gull species are often referred to as 'sea gulls' or 'seagulls'; however, this is a layperson's term and is not used by most ornithologists and biologists. The name is used informally to refer to a common local species (or all gulls in general) and has no fixed taxonomic meaning.^[31] In common usage, gull-like seabirds that are not technically gulls (e.g. albatrosses, fulmars, terns, and skuas) may also be referred to as 'seagulls' by the layperson.

The American Ornithologists' Union combines the Sternidae, Stercorariidae, and Rhynchopidae as subfamilies in the family Laridae, but early 21st-century research^[32][33][34] shows this to be incorrect.

A molecular phylogenetic study published in 2022 found the following relationships between the genera, including the most recent generic change: the placement of Saunders's gull in its own genus Saundersilarus.^[35]

This is a list of the 54 gull species, presented in taxonomic sequence.

Image	Genus	Species
	Larus Linnaeus, 1758	Pacific gull Larus pacificus Belcher's gull Larus belcheri Olrog's gull Larus atlanticus Black-tailed gull Larus crassirostris Heermann's gull Larus heermanni Common gull Larus canus Short-billed gull Larus brachyrhynchus Ring-billed gull Larus delawarensis California gull Larus californicus Great black-backed gull Larus marinus Kelp gull Larus dominicanus ("southern black-backed gull" or "karoro" in New Zealand) Cape gull Larus dominicanus vetula Glaucous-winged gull Larus glaucescens Western gull Larus occidentalis Yellow-footed gull Larus livens Glaucous gull Larus hyperboreus Iceland gull Larus glaucoides Kumlien's gull Larus glaucoides kumlieni Thayer's gull Larus glaucoides thayeri European herring gull Larus argentatus American herring gull Larus smithsonianus Caspian gull Larus cachinnans Yellow-legged gull Larus michahellis Vega gull Larus vegae Armenian gull Larus armenicus Slaty-backed gull Larus schistisagus Lesser black-backed gull Larus fuscus Heuglin's gull Larus fuscus heuglini
	Ichthyaetus Kaup, 1829	White-eyed gull Ichthyaetus leucophthalmus Sooty gull Ichthyaetus hemprichii Pallas's gull (or Great black-headed gull) Ichthyaetus ichthyaetus Audouin's gull Ichthyaetus audouinii Mediterranean gull Ichthyaetus melanocephalus Relict gull Ichthyaetus relictus
	Leucophaeus Bruch, 1853	Dolphin gull Leucophaeus scoresbii Laughing gull Leucophaeus atricilla Franklin's gull Leucophaeus pipixcan Lava gull Leucophaeus fuliginosus Grey gull Leucophaeus modestus
	Chroicocephalus Eyton, 1836	Silver gull Chroicocephalus novaehollandiae Red-billed gull Chroicocephalus novaehollandiae scopulinus Huahine gull Chroicocephalus utunui (extinct) Hartlaub's gull Chroicocephalus hartlaubii Brown-hooded gull Chroicocephalus maculipennis Grey-headed gull Chroicocephalus cirrocephalus Andean gull Chroicocephalus serranus Black-billed gull Chroicocephalus bulleri Brown-headed gull Chroicocephalus brunnicephalus Black-headed gull Chroicocephalus ridibundus Slender-billed gull Chroicocephalus genei Bonaparte's gull Chroicocephalus philadelphia
	Saundersilarus Dwight, 1926	Saunders's gull, Saundersilarus saundersi
	Hydrocoloeus Kaup, 1829 (may include Rhodostethia)	Little gull Hydrocoloeus minutus
	Rhodostethia MacGillivray, 1842	Ross's gull Rhodostethia rosea
	Rissa Stephens, 1826	Black-legged kittiwake Rissa tridactyla Red-legged kittiwake Rissa brevirostris
	Pagophila Kaup, 1829	Ivory gull Pagophila eburnea
	Xema Leach, 1819	Sabine's gull Xema sabini
	Creagrus Bonaparte, 1854	Swallow-tailed gull Creagrus furcatus

The Laridae are known from not-yet-published fossil evidence since the Early Oligocene, some 30–33 million years ago. Three gull-like species were described by Alphonse Milne-Edwards from the early Miocene of Saint-Gérand-le-Puy, France. A fossil gull from the Middle to Late Miocene of Cherry County, Nebraska, US, is placed in the prehistoric genus Gaviota;^[36] apart from this and the undescribed Early Oligocene fossil, all prehistoric species were tentatively assigned to the modern genus Larus. Among those of them that have been confirmed as gulls, Milne-Edwards' "Larus" elegans and "L." totanoides from the Late Oligocene/Early Miocene of southeast France have since been separated in Laricola.^[37]