Gulls and Terns (Laridae)

(Laridae)

Class: Aves

Order: Charadriiformes

Suborder: Lari

Family: Laridae

Thumbnail description
Gulls, skuas, and jaegers are heavy-bodied, long-winged birds with an intermediate length neck and tarsi, webbed feet, and heavy, slightly hooked bills; skimmers are slimmer with a longer, laterally compressed bill, short legs, and a forked tail; terns are smaller and slimmer than gulls; skuas have more strongly hooked bills than gulls; gulls and terns are sometimes listed as a single family, Laridae

Size
Gulls: 10–32 in (25–81 cm), 0.2–4.4 lb (100–2,000 g); terns: 8–22 in (20–56 cm), 0.1–1.7 lb (46–782 g); skimmers: 14–18 in (36–46 cm), 0.2–0.8 lb (111–374 g); skuas and jaegers: 17–24 in (43–61 cm), 0.5–4.6 lb (230–2,100 g)

Number of genera, species
Gulls: 7 genera, 51 species; terns: 10 genera, 44 species; skimmers: 1 genus, 3 species; skuas and jaegers: 2 genera, 7 species (genera treatment varies; some authors consider only 5 genera for terns)

Habitat
High Arctic and sub-Antarctic islands; temperate and tropical seacoasts to interior marshes and deserts; inland rivers

Conservation status
Critically Endangered: 1 species; Endangered: 1 species; Vulnerable: 6 species; Near threatened: 9 species

Distribution
Gulls and terns have a worldwide distribution; skimmers are temperate to tropical; skuas and jaegers are temperate to polar

Evolution and systematics

Many authorities treat the skimmers (Rynchopidae) and the skuas and jaegers (Stercorariidae) as separate families, and some (including the authors of this text) separate the terns (Sternidae) as well. Here, the treatment of these groups is organized as a construction of subfamilies within the family Laridae.

In general, gulls can be divided into the large-white headed and the small dark-hooded tribes. On behavioral grounds, Moynihan treated all gulls in the genus Larus, but most taxonomists separate the unique gulls into their own genera, including the swallow-tailed gull (Creagrus furcatus) of the Galápagos, and several arctic species, among them Ross's gull (Rhodostethia rosea), ivory gull (Pagophila eburnea), kittiwakes (Rissa), and Sabine's gull (Xema sabini). Less often, two South Temperate species—the dolphin gull (Leucophaeus scoresbii) and the pacific gull (Gabianus pacificus)—are separated as well.

The main groups of terns include the crested terns (Hydroprogne, Thalasseus), black-capped terns (Sterna, Gelochelidon), marsh terns (Chlidonias), noddies (Anous, Procelsterna, Gygis), and Inca terns (Larosterna inca). The capped terns include small- and medium-sized birds in the genus Sterna. Sterna (as used here) is a relatively homogenous assemblage. The distinctive, large-bodied, crested terns (Thalasseus) are often placed in Sterna along with some unique capped terns such as the gull-billed (Gelochelidon nilotica), Caspian (Hydroprogne caspia), and large-billed (Phaetusa simplex) terns. Noddies are uniformly either all dark (black, brown, blue, and gray noddies) or all white (white tern). The Inca tern is all dark with dramatic yellow nape wattles.

The three species of skimmers, with their uniquely flattened bill, are closely related to each other and form a superspecies.

Gulls and terns are more closely related than either group is to skimmers, although some taxonomists believe that skimmers are closely related to noddies. Because of their unique bill and foraging method (skimming along the surface of creeks and channels), all species of skimmers are placed in one genus (Rynchops).

Skuas are believed to have derived from gulls, their closest relatives, about 10 million years ago. The three small skuas—pomarine skua (Stercorarius pomarinus), Arctic skua (S. parasiticus), and long-tailed skua (S. longicaudus)—called jaegers in the United States, have similar morphologies, suggesting that they split from the genus Catharacta early on. The three Stercorarius species breed in the Northern Hemisphere, while the large Catharacta skuas breed in the Southern and the Northern hemispheres at high latitudes. The large great skua (C. skua) is presumed to have invaded the Northern Hemisphere from the Southern secondarily.

Physical characteristics

Gulls and terns are white and black, with shades in between. They are generally white below and light gray to black on top. Their white belly is believed to aid in plunge-diving for fish; it serves as camouflage against the pale sky, reducing conspicuousness to their underwater prey. Young birds are generally spotted, blotched, or streaked, affording camouflage on the various substrates they occupy, particularly during the pre-fledging period. Cryptic coloration is essential for the ground-nesting species that occur in large colonies. In all species of gulls and terns, males and females are indistinguishable on the basis of plumage. There is very little sexual size dimorphism.

Gulls are generally white bodied, with a darker mantle varying from pale silvery-gray to black (except for the all-white ivory gull). Several of the smaller species have a pale pink or cream-colored bloom on the breast early in the breeding season; this fades with time. Gulls have either a dark hood or mask or an all-white head during the breeding season. Smaller gulls are generally either dark masked or dark headed. Generally, larger gulls are white headed, although the great black-headed gull is an exception. In almost all species the wingtips are black, the melanin pigment offering resistance to wear. Although several arctic species have white wing tips, most gulls have a complex pattern of white "windows" on the black outer primaries.

Gulls are heavy-bodied, long-winged birds with an intermediate length neck and tarsi, webbed feet, and heavy, slightly hooked bills. Gulls range in weight from 0.2–4.4 lb (100–2,000 g). All species have 12 rectrices (tail feathers), and the tail is rounded in all but a few species. Most gulls molt their flight feathers twice a year and their body feathers once a year. Adult plumage fills in at two to five years.

Most species of terns forage by plunge-diving for fish, and accordingly their bodies are streamlined. Terns have narrower, more elongated bodies than gulls and proportionately longer, more slender, and pointier wings. Their bills are generally slender and sharply pointed. Most terns are white below and gray above, with a black crown in nuptial plumage, although a few species are all dark or all white. Like some smaller gulls, the roseate tern has a delicate pink bloom on its breast that is very pronounced early in the season and is persistent until lost by wear. Terns exhibit no sexual differences in plumage patterns. Weight is 0.1–1.7 lb (46–782 g). Species reach adult plumage in two to six years.

Skimmers are heavy bodied with very long narrow wings and large laterally compressed or knife-like bills for skimming the water. The lower mandible is longer than the upper. Skimmers are the size of the large terns and are generally black above and pure white below, although there is a pale creamcolored tinge, particularly on the flanks, early in the nesting season. Females are about a third smaller in size and weight than males. Species range from 0.2–0.8 lb (111–374 g). The Indian skimmer (Rynchops albicollis) has a broad white collar, and the other two species (black skimmer, Rynchops niger, and African skimmer, Rynchops flavirostris) gain such a collar in their post-breeding molt. The bill is bright reddish-orange with a yellow tip in the African and Indian skimmers; it is red at the base with the distal half black in the black skimmer. Young birds are a whitish-gray with darker gray splotches, making them blend in with their sand substrates.

Skuas and jaegers are similar in shape to gulls but have a heavier bill that is more strongly hooked. Stercorarius have both a light and a dark color phase, and the adult has two elongated central tail feathers. Females are larger than males, as is typical of terrestrial birds of prey. Juvenile skuas are generally darker than adults and have shorter, more rounded wings. Skua flight powers are more highly developed than those of gulls. This is necessary for their piracy behavior. The sharp, hooked bill and sharp claws of a skua help dismantle prey quickly.

Distribution

Gulls and terns are the most characteristic group of birds found along coastal regions in much of the world. Skimmers are restricted to coastal and riverine habitats in North and South America, Africa, and Asia. Skuas are temperate and polar. Although many species nest primarily along coastal regions on beaches or cliffs, others nest in marshes, on the tundra, or in inland lakes and rivers. The Indian skimmer breeds in southern Asia from Pakistan to Cambodia; the African skimmer breeds along rivers in Africa; the black skimmer breeds in North and South America. Gulls tend to concentrate near industrialized and heavily populated regions where there is abundant garbage, offal (fish or animal trimmings), or other sources of food. The ready availability of rubbish contributed to population explosions of several temperate species in the twentieth century. Three species of Stercorarius skuas and one Catharacta inhabit the Northern Hemisphere; three Catharacta species live in the Southern Hemisphere.

Habitat

There is wide variation in the habitat preferences of gulls, terns, skimmers, and skuas, although most feed and nest in association with water. While gulls nest and feed over a wider range of habitats, terns generally forage over water. This foraging constraint of terns somewhat limits their nesting distribution. Ground-nesting gulls and terns nest in places such as coastal or offshore islands that are inaccessible to predators. Those species that do not nest on islands nest on cliffs, in trees, or build floating nests over water, rendering them less susceptible to predators.

Colonies of nesting gulls can be found in coastal and estuarine habitats, as well as on large inland lakes. A few nest mainly on inland lakes or marshes, while two, the lava and swallow-tailed gulls (Larus fuliginosus and Creagrus furcatus, respectively) nest on remote oceanic islands, the Galápagos. Gulls nest in a wide variety of habitats, including sandy or rocky islets, beaches, marshes, river or lake sandbars, windswept sand dunes and cliffs, trees, and even buildings. Kittiwakes nest on cliff ledges or buildings, and Bonaparte's gulls (Larus philadelphia) normally nest in trees. Gray gulls (Larus modestus) uniquely breed in the barren, montane deserts of Chile, flying each day over the Andes to the Pacific Coast to obtain food for their young.

During migration gulls fly to coastal and estuarine habitats, and in winter they generally remain along coasts or on large lakes. Outside the breeding season, gulls are found at virtually all latitudes where open water is available.

Terns also occur throughout the world and breed on all continents. They occupy a wide range of breeding habitats, including inland and coastal marshes; islands in oceans, rivers, lakes, and estuaries; sandy or rocky beaches; and on cliffs and in trees. Terns usually nest on the ground in remote or inaccessible places to avoid mammalian predators. When not breeding, most species migrate to coastal estuaries and the open ocean, although a few never leave their inland marshes and rivers.

Skimmers generally nest on sandy beaches along coasts, or on sandy riverine islands, although some black skimmers now nest on wrack (marine vegetation) in salt marshes.

The three species of Stercorarius breed mainly on the tundra in the Northern Hemisphere, while skuas inhabit a full range of marine habitats during the non-breeding season. The great, brown, and Chilean skuas (Catharacta skua, C. lonnbergi, and C. chilensis, respectively) nest on grass on islands, while the South Polar skua (Catharacta maccormicki) nests inland on snow-free mountain sites where there are breeding petrels and penguins that provide a source of eggs and chicks.

Behavior

Gulls, terns, skuas, and jaegers generally perform most of their foraging, breeding, and migrating during the day. The only gulls that are primarily nocturnal during the breeding season are the swallow-tailed gull of the Galápagos Islands and the gray gull that breeds in the deserts of northern Chile. Skimmers are the most nocturnal and have specialized eye structures to allow feeding and other activities at night.

Gulls and terns breed in either monospecific colonies or in monospecific groups within colonies that include other species. Some species such as Bonaparte's gull breed in very loose, small colonies with fewer than a dozen individuals scattered 0.6 mi (1 km) or more apart. Terns are more gregarious than gulls, and they generally breed, forage, and migrate in flocks that can range up to many thousands or even millions. Most gulls and terns in temperate zones breed at the same time of year, once a year, every year. Some tropical terns have shorter breeding seasons and can breed every eight months. In all species the territory is confined to the area immediately around the nest. Territory size generally increases with body size for gulls and decreases with body size for terns.

Gulls and terns of several species are very aggressive at mobbing potential predators, including human intruders. Although mobbing deters most avian predators, it is less effective against mammals. Species that nest on the ground frequently nest over water or on inaccessible islands to avoid mammalian predators.

Terns are often more active in early morning and late afternoon and sleep at midday. The daily activity patterns of coastal-nesting species of terns are often influenced by tidal cycles. At dawn terns leave their overnight roost or nest site in search of food. These feeding flights may involve large flocks or prolonged streams of individuals and small groups. Following the breeding season terns leave the colony with their chicks and disperse for a few weeks. During this time the chicks practice plunge-diving but obtain most of their food from their parents. Following dispersal terns may gather in flocks of hundreds or thousands to migrate south; some northern species have some of the longest migration routes known.

Skimmers are unique in that they are largely nocturnal, although they do perform some activities during the day. Like gulls and terns, skimmers are highly social, either nesting in small to large conspecific colonies or within heterospecific colonies of gulls and terns. They generally forage solitarily or in small groups. Members of a pair usually face in opposite directions, increasing vigilance. When it is hot, adult and young skimmers often dig in the sand until they reach cooler temperatures.

Unlike gulls, terns, and skimmers, skuas and jaegers are not as highly social as nesting or foraging birds. Over the open ocean they frequently forage alone, although they will join foraging flocks over fish schools.

Feeding ecology and diet

Gulls have diverse foraging behavior and foraging habitats, and feed on a great variety of foods. They are important scavengers. Terns primarily plunge-dive or hover-dip for fish, and skimmers skim the surface of the water for fish; neither scavenges. While gulls are generalist foragers, terns have a more restricted diet; many feed exclusively on fish, while others also forage on insects. Skuas are predators on other seabirds, scavengers around marine mammals, and feed also by piracy on terns, gulls, auks, and other marine birds. Terns are usually limited to feeding over water, while gulls feed on land, along the shore, and over water. Gulls, terns, and especially skuas engage in piracy. In many temperate places, terns and skimmers have been displaced by increasing gull populations that have extended their ranges into new geographical regions and new habitats.

Gulls forage in a variety of habitats, including the open ocean, the surf zone, intertidal mudflats, rivers and rivermouths, rocks and jetties, estuaries, bays, lakes, reservoirs and rivers, wet meadows and farm fields, sewage outfalls, refuse dumps, and even in the air. Many species feed along the shore on a variety of fish and invertebrates. Gulls are particularly characteristic of the intertidal zones. They also feed in a variety of human-influenced situations, including on landfills, behind farm equipment or boats, and by pan-handling from people at fast-food places or along the shore. They forage using a wide range of techniques, including walking on the ground, swimming in the water and dipping for food, and plunge-diving. They also drop hard-shelled animals from some height (33–66 ft [10–20 m]) to crack the shells and thus obtain food. In some species, individuals have specialized diets or foraging techniques when compared to their populations as a whole.

Terns generally dive for fish, either searching on their own or feeding over schools of predatory fish or marine mammals that force forage fish to the surface. Skimmers have a unique foraging method: they skim along the surface of the water, with the lower bill plowing below the water surface, for 82–328 ft (25–100 m) and then fly up, turn around, and often skim back in the other direction. Skimming is facilitated by a laterally-compressed bill. Since theirs is a tactile method of foraging, skimmers are highly crepuscular and nocturnal. Foraging at night also reduces competition with other fish-eating species that nest nearby. Skuas and jaegers use many different foraging behaviors, although they are legendary as pirates and predators on other seabirds. The larger Catharacta species prey on murres, larger gulls, and penguins. When food is scarce, they will feed on berries and carrion. They are opportunistic, and individual birds often specialize on a particular prey species.

Reproductive biology

Most gulls, terns, and skuas in temperate and polar zones breed at the same time of year, once a year, every year. While North American skimmers do likewise, skimmers that are dependent on the formation of sandbars and sandy islands in rivers must wait until such sandbars are exposed. Some tropical terns have shorter breeding seasons and can breed every eight months. All species are monogamous. Both members of the pair engage in territory defense, incubation of eggs, and care of the young. The eggs of most species are brown with dark splotches (skimmers have a whitish-ground color), and clutch size is two or three. Courtship feeding of the female by the male, selection of a nest site, and nest construction precede copulation. Incubation period is 20–30 days; brooding or pre-fledging phase is four to six weeks. Pairs that lose eggs or chicks may initiate a repeat nesting attempt. Following the breeding season, young gulls, skimmers, and skuas may remain with their parents for a few days or weeks. Young terns remain with their parents for weeks or months, perfecting the difficult task of plunge-diving. Some terns migrate with their parents and remain with them much of the winter.

Most gulls breed in colonies of a few to several hundred pair. Several other species, including grebes, ducks, cormorants, herons and egrets, gannets, alcids, shorebirds, and even penguins, nest within or adjacent to gull colonies. Although such species risk predation by nearby nesting gulls, they are protected from other predators by the defending gulls. The nesting period usually lasts three to five months and occurs at the same time each year. An exception to the annual cycle is the swallow-tailed gull of the Galápagos, which breeds in all months of the year. For gulls in general, incubation averages 24–26 days. The first two chicks usually hatch within a few hours of one another, but the third chick may hatch a day or two later, giving it a distinct disadvantage when competing with siblings for food. The cryptic downy chicks are usually a pale gray or pale tan with dark splotches, and they blend in with their nest site. The chicks are brooded until they are a week to two weeks old and usually are guarded until they fledge. Unless disturbed by storms, floods, predators, or humans, gulls use their nests until the chicks can fly. The pre-fledging period in gulls ranges from four to seven weeks, depending on the size of the species. For some gulls parental care extends for a few weeks post-fledging.

In all species of gulls studied, pair-bonds are monogamous, with a high degree of mate fidelity from year to year. However, separation occurs when pairs are unable to work out incubation and brooding activities, when they are unsuccessful at raising offspring, or when one fails to return to the nest site. Paris that remain together lay earlier and raise more young than newly mated pairs. Some promiscuity occurs in gulls, with both sexes copulating with birds other than their mates.

Terns breed in traditional colony sites that may be used for several years or decades. Length of the breeding season varies (two months for arctic species, three to four months for temperate species, and three to five months for tropical species). Chicks remain with their parents for many weeks or months, slowly decreasing their dependence on their parents for food. Nesting colonies of terns range widely in size from a few widely dispersed pairs to dense colonies of a million or more. Intermediate-sized terns usually nest in colonies of tens to hundreds of pairs, with a few colonies exceeding a thousand and some individuals nesting solitarily.

Most terns do not construct a nest but merely make a scrape in the sand, roll a few pebbles or shells around a scrape, or find a suitable cup-shaped place in the coral or rock for their eggs. Terns nesting in marshes construct a nest of vegetation on which eggs can float up if flooding occurs. White terns make no nest, but place their egg on a branch, ledge, or artificial object where it is can easily fall. Clutch size in terns varies from one to three eggs. In low food years, terns reduce the average clutch size, delay breeding, or forego breeding. Members of pairs share incubation duties and chick care, although females may incubate more often, and males may bring back more food.

Skuas and jaegers are monogamous and pair for life, are strongly territorial, and show high nest-site fidelity. Pomarine jaegers are an exception: their life cycle is partly dependent on lemming populations, and their cycle varies accordingly. Skua pairs meet on the territory and re-establish the pair bond each spring. Both sexes form the nest scrape following mutual displays and engage in incubation. Eggs are laid at two to three day intervals, and a clutch usually consists of two eggs. Hatching is asynchronous, allowing the larger and more aggressive chick to survive in poor-food years. Siblings are quite aggressive, leading to considerable wandering about the nest. Following any loss of the clutch, eggs are relaid, particularly early in the breeding season.

Conservation status

Because gulls, terns, and skimmers often breed in coastal regions, their breeding and foraging habitat is increasingly threatened as more and more people concentrate along coasts. Of the 98 species of gulls, terns, and skimmers, world population estimates range from a few hundred pairs (lava gulls, Larus fuliginosus) to several million pairs (herring gulls, Larus argentatus) and several tens of millions (sooty terns, Sterna fuscata). Chinese crested terns (Thalasseus bernsteini) are Critically Endangered. No skuas or jaegers are threatened or endangered, largely because of the low density of human populations at high latitudes.

Egging (collecting bird eggs for food), hunting, and exploitation for the millinery trade resulted in sharp declines of many species in the last two centuries. Human persecution has not ceased in all parts of the world. Current threats include habitat loss, habitat degradation, increased predation (often caused by cats and other animals introduced and maintained by humans), and overfishing by humans that reduces food supplies. Populations are also threatened by pollution, particularly oil spills that occur near nesting colonies or in favorite foraging grounds. Coastal populations are threatened by contaminants in runoff and from rivers. In the 1960s and 1970s egg shell thinning due to DDT was a problem, and in the 1980s in the Great Lakes of North America, organochlorines contributed to decreased hatching rates, lowered parental attendance, lower reproductive success, and congenital defects.

In many temperate regions, the large white-headed gulls expanded their numbers and ranges dramatically during the twentieth century, abetted by the availability of human refuse in uncontrolled garbage dumps. This new food source greatly increased the survival of juvenile gulls. The large gulls displaced smaller gulls and terns from their traditional nesting sites. They also preyed on the eggs and chicks of these species. New landfill practices and alternative refuse management has reduced this food source in many urban areas, and the populations of some gulls have begun to decline.

Conservation measures include protecting colonies from direct exploitation (hunting of adults, egging); creating suitable, secure nesting space; stabilizing ephemeral nesting habitats; constructing artificial nesting islands or platforms; removing predators (feral cats, large gulls, foxes); protecting from other predators; and reducing human disturbance at colony sites through sign-posting, fencing, or even wardening. More difficult but equally important is the protection of foraging sites and the prey base, which may involve fisheries management. For native human populations, measures must be instituted that allow sustained egging while protecting seabird reproduction.

Significance to humans

Feathers and whole bodies were used in the late 1800s for decorations on ladies' hats. In many parts of the world eggs are still collected for food, and the eggs of some species are considered an aphrodisiac. Meat may also be eaten by some peoples. Terns are used by fishers to guide them to flocks of predatory fish, and both gulls and terns were used by early mariners to indicate the presence of land. Because they mainly nest in tundra habitats, skuas and jaegers have been relatively unaffected by humans.

Species accounts

Resources

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 3, Hoatzin to Auks. Barcelona: Lynx Edicions, 1996.

Furness, R. W. The Skuas. Stratfordshire, UK: T. & A.D. Poyser, 1987.

Grant, P. J. Gulls: A Guide to Identification. Vermillion, SD: Buteo Books, 1997.

Olsen, K. M., and H. Larsson. Skuas and Jaegers of the World. Princeton, NJ: Princeton University Press, 1997.

Olsen, K. M., and H. Larsson. Terns of Europe and North America. Princeton, NJ: Princeton University Press, 1995.

Schreiber, B. A., and J. Burger. Biology of Marine Birds. Boca Raton, FL: CRC Press, 2001.

Tinbergen, N. The Herring Gull's World. New York: Basic Books, 1960.

Roberson, Don. "Gulls (Laridae)" Bird Families of the World. 19 Dec. 2001

[Article by: Joanna Burger, PhD; Michael Gochfeld, PhD, MD]