Gulls, often referred to
as seagulls,[1][2] are seabirds of the family Laridae in
the suborder Lari. They are most closely related to theterns (family Sternidae) and only distantly
related to auks, skimmers, and more distantly to
the waders. Until 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
severalgenera.[3] An
older name for gulls is mew, cognate with
German Möwe, Danish måge,
Dutch meeuw, and French mouette; this
term can still be found in certain regional dialects.[4][5][6]
Gulls are typically medium to large birds, usually grey or white,
often with black markings on the head or wings. They typically have
harsh wailing or squawking calls, stout, longish bills, and webbed feet. Most gulls,
particularly Larus species, are
ground-nestingcarnivores, which take live food
or scavenge opportunistically. Live food
often includes crabs and small fish. Gulls have unhinging jaws which allow them
to consume large prey. Apart from the kittiwakes, gulls are typically coastal or
inland species, rarely venturing far out to sea.[7] The
large species take up to four years to attain full
adult plumage, but two years is typical for small
gulls. Large white-headed gulls are typically
long-lived birds, with a maximum age of 49 years recorded for
the herring gull.[8]
Gulls nest in large, densely packed noisy colonies. They lay two or three
speckled eggs in nests composed of vegetation. The young
are precocial, being born with dark
mottled down, and mobile upon hatching.[9]
Gulls—the larger species in particular—are resourceful,
inquisitive and intelligent[10] birds,
demonstrating complex methods of communication and a highly
developed social structure. For example, many gull colonies
display mobbing behaviour, attacking and
harassing would-be predators and other intruders.[11] Certain
species (e.g. the herring gull) have exhibited tool use behaviour, using pieces of
bread as bait with which to
catch goldfish, for example.[12] Many
species of gulls have learned to coexist successfully with humans
and have thrived in human habitats.[13] Others
rely on kleptoparasitism to get their
food. Gulls have been observed preying on live whales, landing on the whale as it surfaces to
peck out pieces of flesh.[14]
Description and
morphology[edit]
The
Pacific gull is a large white-headed
gull with a particularly heavy bill.
Gull species range in size from the little gull, at 120 g (4.2 oz)
and 29 cm (11 in), to the great black-backed gull, at
1.75 kg (3.9 lb) and 76 cm (30 in). They are
generally uniform in shape, with heavy bodies, long wings, 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 (certainly longer than the 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.[15]
The gulls are generalist feeders, indeed 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.[15]
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.[15]
Distribution and
habitat[edit]
The gulls have a worldwide cosmopolitan distribution.
They breed on every continent, including the margins
of Antarctica, and are found in the high
Arctic, as well. They are less common on tropical islands, although
a few species do live on islands such as the Galapagos and New Caledonia. Many species breed in
coastal colonies, with a preference for islands, and one species,
the grey gull, breeds in the interior of dry
deserts far from water. Considerable variety exists in the family
and species may breed and feed in marine, freshwater, or
terrestrial habitats.[15]
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, like Franklin´s gull, which migrates from
Canada to wintering grounds in the south of South America. Other
species move much shorter distances and may simply disperse along
the coasts near their breeding sites.[15]
Behaviour[edit]
Diet and feeding[edit]
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, 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.[15]
In addition to taking a wide range 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. A method of obtaining prey unique
to gulls involves dropping heavy shells of clams and mussels onto
hard surfaces.[15] 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.[17] 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.[15]
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.[15] Examples
of such associations include four species of gulls feeding around
plumes of mud brought to the surface by feeding grey whales,[18] and
also between orcas (largest dolphin species)
and kelp gulls (and other
seabirds).[19]
Breeding[edit]
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 areterritorial, 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.[15]
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.[15]
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.[15] 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.[15] Research
on various bird species including the gull suggests that females
form pair bonds with other females to obtainalloparental care for their
dependent offspring, a behavior seen in other animal species, like
the elephants, wolves, and the fathead minnow.[20]
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.[15]
Taxonomy[edit]
The taxonomy of gulls is confused
by their widespread distribution zones of hybridization leading
to geneflow. Some have traditionally been
considered ring species, but recent evidence suggests
that this assumption is questionable.[21] Until
recently, most gulls were placed in the genus Larus,
but this arrangement is now known to be polyphyletic, leading to the resurrection
of the generaIchthyaetus, Chroicocephalus, Leucophaeus, Saundersilarus,
and Hydrocoloeus.[3] Some
English names refer to species complexes within the group:
Hybridisation between
species of gull occurs quite frequently, although to varying
degrees depending on the species involved. The taxonomy of the
large white-headed gulls is particularly complicated.
In common usage, members of various gull species are often referred
to as sea gulls or seagulls; however, "seagull" is a layperson´s
term that is not used by most ornithologists and biologists. This
name is used informally to refer to a common local species or all
gulls in general, and has no fixed taxonomic meaning.[citation
needed] In common usage, gull-like
seabirds that are not technically gulls (e.g. albatrosses, fulmars, kittiwakes, terns, and skuas) may also be referred to as seagulls by the
layperson.
A gull at Point Lobos State Natural Reserve, CA, USA
The American Ornithologists´
Union combines the Sternidae, Stercorariidae, and Rhynchopidae as subfamilies in the family
Laridae, but recent research[22][23][24] indicates
this is incorrect.
List of species[edit]
Gulls acquire food from humans both through handouts
and theft
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