This picture of decline underlies a more pressing conservation crisis, with 11 per cent of species now facing imminent extinction. Both issues were highlighted this week by Birdlife International, a global bird protection organisation, at a conference in Rosenheim, Germany.
Christoph Imboden, director general of the organisation, said 1,111 species were on his threatened list. The fall in bird populations was a strong indicator of pressure on all plant and animal species, and the latest figures should serve as a warning of the destructive impact of mankind's behaviour on global biodiversity, he said.
Birdlife International, of which the Royal Society for the Protection of Birds is a partner, said analysis of all species in Europe had found that in the past 20 years, 38 per cent, including those thought to be widespread such as skylarks and tree sparrows, have declined. 'In a quarter of these, the decline is noticeable in more than 50 per cent of the populations,' Mr Imboden said.
The tree sparrow has suffered an 80 per cent drop in numbers in the past 20 years. 'We tend to focus on the ones already threatened, but this is another dimension to the loss,' he said. Full details will be published in a report by the organisation due out in November.
The biggest threat is the increase in human population and the loss of natural habitats this brings with it. Mr Imboden said that in Europe, it is a question of changing land use and agricultural practices with more intensive farming in some areas and more land set aside in others. This increases the chances for some species, but threatens far more.
The only British native bird on the list is the corncrake. Modern farming methods have reduced pairs from more than 2,500 to just 479 in the past 25 years. It can now only be found in the remote crofting communities of the Outer Hebrides.
Wading birds are also at risk. The black-tailed godwit and the redshank have been hit badly. The RSPB wants the Government to use some set-aside land to create new habitats for such birds.
More than 100 species could disappear within five years. These include some types of parrot, cranes, eagles, ibis and seabirds such as the Galapagos penguin.
The world's most endangered bird of all is the Spix's macaw. There is believed to be only one surviving in its natural habitat - riverside forests in Brazil. Also threatened is its cousin, the hyacinth macaw, the largest of all parrots. There are only 1,500 pairs in existence in the Brazilian rainforests.
Another parrot, the flightless kakapo found in New Zealand, has been preyed on by cats and there are thought to be only 50 left. In Thailand, Gurney's pitta has declined as a result of deforestation and trapping, and only about 30 are thought to remain.
That the persistent decline should be so widespread is a strong biological indicator of pressure on other plant and animal species, the conference was told.
'These figures are important not just to the bird conservation community, but to the conservation community in general. Birds are representative of what is happening to all animal species,' Mr Imboden said. He estimates that from his study of birds, up to 10 million plant and animal species could be under threat globally.
'We have found that in areas of greatest accumulation of unique bird species there is a high degree of cogruence with unique representatives of plant and animal species in general,' he said.
At least half of the globally threatened species inhabit tropical forests.
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