Gull

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 opportunistically take a wide range of prey. 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, plant items such as seeds and fruit, human refuse, chips,[17] 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, and terrestrial prey such as seeds, fruit, and earthworms are 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]

In addition to taking a wide range of prey, gulls display great versatility in how they obtain prey. Prey can be obtained in the air, on water, or on land. In the air, a number of hooded species are able to hawk insects on the wing; larger species perform this feat more rarely. Gulls on the wing also snatch items both off water and off the ground, and over water they also plunge-dive to catch prey. Again, smaller species are more manoeuvrable and better able to hover-dip fish from the air. Dipping is also 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. In shallow water gulls may also engage in foot paddling.[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 apparently a learned component to the task exists, as older birds are more successful than younger ones.[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 to feed on deeper prey. To obtain prey from deeper down, 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 feeding around plumes of mud brought to the surface by feeding grey whales,[20] and also between orcas (largest dolphin species) and kelp gulls (and 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 behavior. Analysis of yellow-legged gull's (Larus michahellis) pellets off the northwest coast of Spain 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, furthering creating a stress on their shift in diet.[22] Between the years of 1974–1994, yellow-legged gull populations in 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 a marine to terrestrial prey highlight the resilience adult gulls have and their ability to keep chick condition consistent.[23] Human disturbance has also 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 eye balls of baby seals, and directly pilfer milk from the elephant seal's teat.[25][26]

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  American herring gull eating a starfish
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  Gull attacking a coot – this gull is probably going after the bread or other food item in the bill of this American coot, though great black-backed gulls are known to kill and eat coots
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  Lesser black-backed gulls in a feeding frenzy
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  Juvenile ring-billed gull at Sandy Hook, New Jersey
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  European herring gull stealing food from a man's hand

Black-legged kittiwakes nest colonially, but have tiny, closely packed territories

Gulls are monogamous and colonial breeders that display mate fidelity that usually 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 in the same location within that colony. 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 birds. 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-m radius around the nest in the 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 the colony. Existing pairs re-establish their pair-bonds, and unpaired birds begin courting. Birds 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 through calls and aerial attacks.[13]

The nest of a great black-backed gull, with three typical eggs

Nest building is also part of the pair-bonding. Gull nests are usually mats of herbaceous matter with a central nest cup. Nests are usually built on the ground, but a few species build nests on cliffs, including the kittiwakes, which almost always nest in such habitats, and in some cases in trees, and high places like Bonaparte's gulls. Species that nest in marshes must construct a nesting platform to keep the nest dry, particularly in 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 it is two in some of the smaller species and only one egg for the swallow-tailed gull. Within colonies, birds synchronise their laying, with synchronisation being higher in larger colonies, although after a certain point, this levels off. The eggs of gulls are usually dark tan to brown or dark olive with dark splotches and scrawl markings, and are well camouflaged. Both sexes incubate the eggs, with incubation bouts lasting between one and four hours during the day and one parent incubating through the night.[13] Research on various bird species including the gull suggests that females form pair bonds with other females to obtain alloparental care for their dependent offspring, a behavior seen in other animal species, like the elephants, wolves, and the fathead minnow.[27]

Incubation lasts between 22 and 26 days, and begins after laying the first egg, although it is discontinuous until the second egg is laid. This means the first two chicks are born close together, and the third chick some time later. Young chicks are brooded by their parents for about one or two weeks, and often at least one parent remains with them, until they fledge, to guard them. Both parents feed the chicks, although early on in the rearing period, the male does most of the feeding and the female most of the brooding and guarding.[13]