Birds

Defining marine birds, or seabirds, is difficult because all birds breed on land, but a large number obtain all or much of their food from aquatic marine or littoral habitats. Some of these, including all frigatebirds (Fregatidae), tropicbirds (Phaethontidae), gannets and boobies (Sulidae), penguins (Spheniscidae) and petrels, albatrosses and shearwaters (Procellariidae) are indisputably marine, in that they obtain all their food from marine habitats, almost invariably breeding along coastlines and spending most or all of their time when not breeding out at sea. Many others, however, have less well defined habits. Some, such as a number of cormorants and shags (Phalacrocoracidae) have both resident inland and marine populations. Others, such as a number of gulls and terns (Laridae) and some ducks and geese (Anatidae), may breed inland but spend the rest of the year living in coastal areas or out at sea. Yet others, such as sandpipers (Scolopacidae) and other waders, typically feed in littoral or intertidal habitats rather than in the sea itself. Many of these species also occur inland.

Somewhat arbitrarily, and excluding all wading birds with the exception of the red phalarope Phalaropus fulicaria (a truly pelagic species outside the breeding season), over 300 species of birds can be considered wholly or largely marine. These species show a latitudinal distribution in which diversity is much higher at higher latitudes than it is in the tropics. Two thirds of all seabirds are confined as breeding species to these latitudes, compared with only 7% that are exclusively tropical.

Diversity is also markedly higher in the southern than in the northern hemisphere, with over half of all seabird species breeding in southern temperate and polar latitudes. Dominance of this region is even more marked in the Procellariidae, the family with the greatest number of truly marine species, in which over 60% of species breed at these latitudes and half are confined to it.