seabirds. There are over 300 species of birds belonging to around sixteen families that rely on the sea for their food and only return to land to nest. Many, including most gulls (44 species), are restricted to coastal waters, but several are truly oceanic, and even when nesting, fly hundreds of kilometres to collect food. There are four main modes of feeding: diving, plunging, surface feeding, and stealing from other seabirds.
Diving is best developed in the seventeen species of
penguins. Penguins are restricted to the southern hemisphere and in the northern hemisphere their feeding niche is filled by the eighteen species of
auks and three species of puffins (
Fratercula spp.), which can still fly, but also use their wings to swim underwater, which restricts their size. The 26 species of cormorant (
Phalacrocorax spp.) also dive for
fish using both their feet and their wings to swim underwater. One cormorant, the Galapagos cormorant (Phalacrocorax harrisi), has become flightless, a characteristic of island birds. The nine species of gannets and
boobies (
Sula spp.) are plunge divers and eight species of pelican (
Pelecanus spp.) mix plunge diving with surface fishing. The sight of a large flock of gannets over a
shoal of fish hurtling into the water at high speed is one of the more dramatic sights in inshore waters. The 40 species of terns (
Sterna spp.) and noddies (e.g.
Anous spp.), also known as sea swallows, also plunge dive. Terns are remarkable strong fliers: the Arctic tern (Sterna paradisaea) undertakes the longest migration of any bird, commuting from its nesting grounds in Arctic to overwinter in the Antarctic, a round trip of about 35,000 kilometres (22,000 mls.), and so enjoys almost continuous summer and almost continuous daylight.
Surface feeding, often along
slicks, is the mode used by the greatest number of seabirds. They tend to be more nocturnal, because at night many species of
plankton and small
fish migrate up to the surface. Some of these surface feeders are strong fliers, using gliding flight to cover vast distances with minimum expenditure of energy. The most efficient of these gliders are the thirteen species of
albatrosses and
mollymawks.
There are three species of sea eagle (
Haliaetus spp.), which, together with the bald eagle (Haliaetus leucocephalus) and the osprey (Pandion haliaetus), can snatch fish out of the water with their talons. They are also scavengers, eating the bodies of spent salmon and following fishing boats to pick up their discards. The Steller's sea eagle (Haliaeetus pelagicus) is the largest of all eagles and occurs along the north-west Pacific coast.
Many of the 61 species of shearwaters (
Puffinus spp.),
fulmars, prions, and petrels (Procellariidae) are also efficient gliders, effortlessly quartering the seas exploiting the updrafts of the waves and rarely having to resort to flapping flight, whereas the sixteen species of storm petrels (Hydrobatiidae), or
Mother Carey's Chickens, flutter almost ceaselessly over the troughs and crests of the
waves, picking up the tiniest of plankton from the surface.
The 42 species of gulls (Lariidae) tend to be surface feeders and scavengers, often following fishing boats in large numbers waiting for the offal to be discarded. Recently many gull species have become more terrestrial, particularly during winter exploiting our rubbish tips and following the plough. Some gulls are highwaymen, harassing other birds to disgorge the food they have collected, and also indulging in egg predation and eating nestlings. The most efficient of these piratical birds are the seven species of skuas and jaegers (Stercorariidae) who have been given a variety of other names by mariners, such as bonxies and aulin. Every nesting colony of seabirds is attended by parties of these piratical raiders, and they are frequent visitors to seal and sea–lion colonies where they scavenge on corpses, and eat afterbirths, regurgitated food, and even faeces. The two species of giant petrel (Macronectes sp.) are also scavengers, and the three species of frigate, or man-of-war, birds (Fregatta spp.) are acrobatic aerial highwaymen harrying boobies to disgorge the contents of their crops. Perhaps the weirdest feeding mode is that of the two sheathbills (
Chionus spp.) that walk around penguin colonies feeding almost exclusively on penguin faeces.
Seabirds are vulnerable to predation when nesting, so they often nest on remote islands or sea cliffs, where they can find a degree of safety from predators, such as rats, cats, foxes, and even man. Many shearwaters have also become nocturnal visitors to their nests, so avoiding being harried by the aerial pirates. The three tropic birds (
Phaethon spp.) nest far inland on mountainous islands, but the most extreme precaution is taken by the snow petrel (Pagodroma nivea), which nests deep in the icy interior of Antarctica.
Bibliography
Harrison, P. , Seabirds: An Identification Guide (1983).
Nelson, B. , Seabirds: Their Biology and Ecology (1981).
M. V. Angel
