Due primarily to the impact of man on the natural environment, whether directly or indirectly, many of the 8,590 species of birds are in danger of extinction. Pesticides kill them by direct poisoning and through accumulation in the food chain, or affect their reproductive capacity by causing thin and fragile egg shells or poisoned embryos. They suffer destruction by hunters and trappers including poisoning by lead shot. They are killed by poison bait left for other animals. Nests and sources of food can be destroyed by intensive farming techniques and by destruction of natural habitats through over-grazing, irrigation, desertification, and forest loss. Construction, urbanization and draining swamps and marshes also destroy natural habitats.
In 1600 (when reliable zoological records began) an estimated 8,684 species of bird were extant. Of these, 94 (or 1.09%) have since become extinct. A further 187 species (or 2.16%) are now in extreme danger of extinction, and around 10% are endangered (threatened with extinction). 164 subspecies have become extinct and a further 287 are in danger. Around 50% of all bird species are noticeably declining in numbers. Some species are becoming more widespread; but most are declining in number and range. In some areas, exotic birds and birds that can thrive in a disturbed habitat are becoming more common at the expense of native birds.
IUCN Conservation and Management Plan (CAMP) assessments of threat to certain bird taxa (available in 1993) found for Anseriformes (ducks, geese and swans): 66 species or 28% threatened; cranes: 23 species (74%) threatened; parrots 139 species (32%) threatened; and Asian hornbills: 44 species (85%) threatened. Species "critically threatened" (population under 250; 50% probability of extinction within 5 years or 2 generations, whichever is longer) for the above taxa were: Anseriformes: 10 species cranes: 9; parrots 25; Asian hornbills 5.
A 1992 study by the International Council for Bird Preservation found 221 areas in the world that have high concentrations of birds and which also have about two thirds of the world's 1,029 endangered species. Most of the 'hotspot' sites, amounting to 260,000 square kilometres, are in forest regions of the tropics, with about a quarter in South America. Forty two are in Australasia, 40 in Africa or on African islands, and 30 on Pacific islands. One site is in Europe, on Cyprus.
Countries with the greatest number of bird species immediately under threat of extinction are: the Philippines with 126 species, Brazil 97, China 81, Peru and Columbia more than 50. 280 out of 530 European species are vulnerable and more than one third have declined in numbers, some dramatically, since 1975, for example the aquatic warbler, the imperial eagle and the pallid harrier. Once widespread in the north and west of Britain, The range of the corncrake in Britain has contracted more than two-thirds in 20 years. It is now virtually confined to the Scottish islands and part of Ireland and could be extinct in the British Isles within another 20 years. Its decline mirrors the falls for many of the other 28 predominantly farmland species (such as the corn bunting, grey partridge and yellowhammer) all but four of of which have declined significantly in range and numbers due to intensive agricultural practices.