Gulls, Terns, and Relatives: Laridae

views updated

GULLS, TERNS, AND RELATIVES: Laridae

ARCTIC SKUA (Stercorarius parasiticus): SPECIES ACCOUNTS
SAUNDER'S GULL (Larus saundersi): SPECIES ACCOUNTS
BLACK TERN (Chlidonias niger): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

Gulls, terns, and their relatives vary between 8 and 32 inches (20 to 81 centimeters) in length and between 1.6 and 74 ounces (46 to 2,100 grams) in weight. Gulls and terns generally have white bellies and gray or black backs. Males and females are similar in both size and coloration. Young birds, however, are usually spotted or streaked to help them blend in with their environments. During the breeding season, some gulls develop a pink or cream colored patch on the breast. Gulls have heavy bodies and long wings. Terns have narrower, longer bodies and long, slender, pointed wings. Their bills are slender and pointed. Many terns develop a black crown on top of the head during the breeding season.

Skimmers have heavy bodies and long, narrow wings. Their bills are large and laterally compressed, flattened from left to right. In skimmers, males are often significantly larger than females. Most skimmers have black backs and white bellies, although during the breeding season the legs may become cream colored.

Skuas and jaegers (YAY-gerz) have body shapes similar to that of gulls, but have heavy, hooked bills. They are exceptionally powerful fliers. Females are larger than males.

GEOGRAPHIC RANGE

Gulls and terns are found in coastal regions worldwide. Skimmers are found in temperate, not too hot or too cold, and tropical regions in North America, South America, Africa, and Asia. Skuas and jaegers are found in temperate and polar areas.

HABITAT

Terns frequently nest on islands or in coastal areas where there are few predators. They may also avoid predators by nesting on cliffs, in trees, or even in the water on floating vegetation. Gulls nest in coastal areas as well as in wetlands or along the shores of large lakes. Gulls use beaches, marshes, river or lakeshores, sand dunes, cliffs, trees, and buildings as nest sites. One species, the gray gull, breeds in the mountainous deserts of Chile and flies over the Andes mountains each day to find food in the Pacific Ocean. During the nonbreeding season, gulls will make use of almost any habitat close to open water. Skimmers nest on coastal beaches or in salt marshes, and move to the open ocean during the nonbreeding season. Skuas breed on tundra or grassy islands. The south polar skua nests near colonies of breeding petrels and penguins.

DIET

Gulls eat a wide variety of foods. Many obtain fish and invertebrates, animals without backbones, from the seashore. Some species take advantage of human garbage or beg for handouts from people. Gulls have a wide variety of methods for obtaining food, including walking on the ground, searching in the water, and diving. They are also known to drop mollusks and other hard-shelled animals from a large height to crack the shells. Terns usually dive in the ocean for fish. Skimmers catch prey by skimming along the water with their lower bills underwater. Skimmers usually forage, search for food, either at dusk or at night. Skuas and jaegers are predatory, feeding on other bird species such as murres, gulls, and penguins.

ASYNCHRONOUS HATCHING IN SKUAS

Female skuas usually lay two eggs at a time. The two skua eggs hatch asynchronously (ay-SIN-kron-us-lee), that is, one egg hatches two or three days before the other. Asynchronous hatching allows the first chick to be larger and stronger than the other. In years where there is little food, parents can usually raise only one chick and so only the first chick survives.

BEHAVIOR AND REPRODUCTION

Aside from skimmers and two species of gulls, most members of the gull and tern family are diurnal, active during the day. During the breeding season, gulls and terns nest in small or large colonies. Terns in particular are found in very large groups that may include millions of individuals. Gulls and terns are territorial during the breeding season, defending a small area around the nest from other individuals of the species. Most gulls and terns will mob potential predators and intruders, with many birds attacking simultaneously. This strategy is very successful against other bird species, but does not work as well with mammalian predators.

Skimmers are nocturnal, active at night. They nest in colonies, sometimes ones that include gulls and terns. The members of a skimmer pair usually face in opposite directions while at the nest to more effectively scan for predators. Skuas and jaegers tend to be found alone, although they sometimes form foraging flocks over schools of fish.

Gulls, terns, and their relatives are monogamous (muh-NAH-guh-mus), with each male breeding with a single female. In many species, individuals keep the same mates from one year to the next. Pairs that stay together are often able to raise more young than newly mated pairs. However, in many gulls, individuals of both sexes sometimes copulate, breed, with birds other than their mates. Both male and female participate in incubating, or sitting on the eggs, defending the territory from intruders, and feeding and protecting the young once they hatch. Females lay one to three eggs at a time. The eggs are usually brown with dark markings. Eggs hatch after twenty to thirty days, and chicks are able to fly after four to six weeks. If the eggs or chicks are lost, particularly early in the breeding season, the female will often lay a new set of eggs. Chicks generally remain with their parents for some time after leaving the nest, particularly among the terns, where young have to learn the difficult art of diving for food. Some young terns will migrate with their parents and spend much of the winter with them.

GULLS, TERNS, RELATIVES, AND PEOPLE

In the 1800s, gull and tern feathers, and sometimes even whole birds, were used to decorate women's hats. The eggs of certain species have been, and continue to be, collected for food. Some eggs are considered aphrodisiacs (aff-roh-DEE-zee-acks), substances that enhance sexual desire, in parts of the world. Adult gulls and terns are also sometimes hunted for food. Gulls and terns were sometimes used as a sign that land was nearby by sailors, and terns are still used to locate schools of fish. Because of their more remote tundra habitats, skuas and jaegers have interacted less with human beings.

CONSERVATION STATUS

Chinese crested terns are Critically Endangered, facing an extremely high risk of extinction. Threats to gulls and terns include habitat destruction and disturbance, hunting, egg collection, predation by cats and other species associated with humans, pollution from oil spills and pesticides.

ARCTIC SKUA (Stercorarius parasiticus): SPECIES ACCOUNTS

Physical characteristics: Some Arctic skuas are entirely brown, while others have a dark gray head, white neck and belly, and dark back and wings.

Geographic range: Arctic skuas are found in far northern coastal areas, near the North Pole, during the breeding season. They spend the winter in the Southern Hemisphere in coastal areas.

Habitat: Arctic skuas breed on tundra, treeless plains found in artic regions, or grasslands. During the nonbreeding season, they occupy ocean areas close to land.

Diet: Arctic skuas eat lemmings and the eggs and chicks of other seabirds.

Behavior and reproduction: Arctic skuas are diurnal, active during the day. They are often found with other birds, particularly alcids (birds in the family Alcidae), gulls, and terns, while breeding and searching for food.

Arctic skuas and people: Arctic skuas are sometimes shot by humans who believe they damage sheep and other livestock.

Conservation status: Arctic skuas are not considered threatened at this time. ∎

SAUNDER'S GULL (Larus saundersi): SPECIES ACCOUNTS

Physical characteristics: Saunder's gull has a black head and neck, white throat and belly, white eye crescents, and gray back. The bill is red with a dark band. Juveniles and nonbreeding adults are primarily white.

Geographic range: Saunder's gulls breed in coastal areas of eastern China and spend the winter in South Korea, southern Japan, and North Vietnam.

Habitat: Saunder's gulls live in coastal wetland areas during the breeding season. In the winter they are generally found in seashore areas.

Diet: Saunder's gulls forage, search for food, along the coast in mudflats as well as in coastal lagoons.

Behavior and reproduction: Other than the fact that they are diurnal, little is known about the behavior of Saunder's gulls. Saunder's gulls form breeding colonies on coastal salt marshes. The female lays three eggs at a time.

Saunder's gulls and people: Local populations collect the eggs of this species for food.

Conservation status: Saunder's gulls are considered Vulnerable, facing a high risk of extinction in the wild. The total population is under 5,000 individuals. Decline has been due primarily to habitat loss for agriculture and oil exploration. ∎

BLACK TERN (Chlidonias niger): SPECIES ACCOUNTS

Physical characteristics: Black terns have black heads, necks, and breasts. Their backs and bellies are dark gray in color. Juveniles and nonbreeding adults are pale gray on the back and white on the belly and head, with a dark patch on the side of the breast.

Geographic range: Black terns are found in temperate North America, Europe, and Eastern Asia during the breeding season. They spend the winter in Central and South America and in Africa.

Habitat: Black terns breed in inland habitats such as ponds, lakes, and marshes. In the winter, they occupy seashore and coastal wetland habitats.

Diet: Black terns eat aquatic insects, snails, small fish, tadpoles, and frogs.

Behavior and reproduction: Black terns breed in small colonies, generally fewer than twenty individuals, although colonies of as many as a hundred birds have been seen. A single male mates with a single female, and both parents help incubate, sit on, the eggs as well as take care of young. The black tern nest is usually built on top of floating vegetation. The female lays two to three eggs at a time, and these hatch after twenty to twenty-three days. Chicks leave the nest after twenty-five days.

Black terns and people: The preferred nesting areas of black terns include the small lakes or marshes that are often drained by humans.

Conservation status: Black terns are not considered threatened. However, some populations have declined due to destruction of wetland habitats, pesticides, and competition with human-introduced fish for food. ∎