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Birds are found worldwide in many habitats. They can fly over some of the highest mountains on earth as well as both of the earth’s poles, dive through water to depths of more than 250 m (850 ft), and occupy habitats with the most extreme climates on the planet, including arctic tundra and the Sahara Desert. Certain kinds of seabirds are commonly seen over the open ocean thousands of kilometers from the nearest land, but all birds must come ashore to raise their young.
Many sea birds are excellent swimmers and divers, including such distantly related types of birds as grebes, loons, ducks, auks, cormorants, penguins, and diving petrels. Most of these sea birds have webbed or lobed toes that act as paddles, which they use to propel themselves underwater. Others, including auks and penguins, use their wings to propel themselves through the water. Swimming sea birds have broad, raft like bodies that provide stability. They have dense feather coverings that hold pockets of air for warmth, but they can compress the air out of these pockets to reduce buoyancy when diving.
Many fish-catching sea birds can dive to great depths, either from the air or from the water’s surface. The emperor penguin can plunge to depths of more than 250 m (850 ft) and remain submerged for about 12 minutes. Some ducks, swans, and geese perform an action called dabbling, in which they tip their tails up and reach down with their beaks to forage on the mud beneath shallow water.
Sea birds obtain most or all of their food from the water. All aquatic sea birds that live in saltwater environments have salt glands, which enable them to drink seawater and excrete the excess salt. Albatross, shearwaters, storm petrels, and diving petrels are considered the most exclusively marine of all sea birds. Sea birds spend much of their time over the open ocean, well away from land.
Many other sea birds have aquatic lifestyles but live closer to land. Among these are penguins, which live in the southernmost oceans near the Antarctic. Some species of penguins spend most of their lives in the water, coming on land only to reproduce and molt. Grebes and divers, or loons, are found on or near lakes. Grebes are unusual among sea birds because they make their nests on the water, using floating plant materials that they hide among reeds. Pelicans, known for their long bills and huge throat pouches, often switch between salt water and fresh water habitats during the year. Gulls are generalists among the sea birds, feeding largely by scavenging over open water, along shores, or even inland areas. Waterfowl, a group that includes ducks, geese, and swans, often breed on freshwater lakes and marshes, although they sometimes make their homes in marine habitats.
Seabirds - Biological Significance
Seabirds have benefited from controls on hunting and egg collecting, which were once prevalent in the nineteenth century for sport, food and feathers. Migratory species are affected by different pressures in each country they visit. For example, the roseate tern faces natural predation and trapping in West Africa, but not in the UK.
Seabirds - Affects and Effects
Seabirds may be affected by:
- The availability of undisturbed nesting habitat
- Human competition for fish
- The amounts of discarded fish from fisheries
- Oiling mortality
- Net entanglement
It is also possible that long-term changes in the climate and oceans have affected populations of both sea birds and the marine fish that they feed on. Oil is one of the greatest hazards because spills initially float, exposing sea birds to oiling of their plumage. The Mersey pipeline oil leak in 1989 killed over 4,000 sea birds, most of which were common species of gull. The Sea Empress spill in 1986 is known to have killed at least 7,000 sea birds. This included some 3,500 over wintering common scoters. Although such pollution has occasionally led to substantial mortalities of sea birds in estuaries, there is no evidence of it having any long-term significance to populations.