Two of Britain's best known birds of prey are undergoing starkly contrasting changes in abundance – one very much up, and the other plummeting.
The differing fortunes of the common buzzard, which is soaring in population, and the kestrel, Britain's commonest falcon but now in sharp decline, are highlighted by the first fieldwork for the mammoth new atlas of the breeding birds of Britain and Ireland.
Under the supervision of the British Trust for Ornithology (BTO), thousands of volunteer observers are plotting the distribution of all of the British Isles' 200-plus breeding species over the next four years, in a vast exercise to update the last atlas, which was published in 1991.
Comparison with the 1991 atlas clearly shows the notable changes in population affecting various species, and this weekend the BTO releases provisional data from this spring's surveys on four of them: the kestrel, the lapwing and the yellow wagtail – all declining in comparison with their 1991 range – and the buzzard, strongly increasing.
The buzzard used to be confined more or less to the west of a line down the centre of England, ending in the Isle of Wight. But in the past 20 years it has spread to the east and is now to be found in significant numbers right across eastern England, from Lincolnshire, East Anglia and down to Kent.
The decline of persecution by gamekeepers may be a reason, as a similar movement has been seen in populations of the raven.
The kestrel, on the other hand, which was widely found over much of Britain, is now much more thinly distributed, especially in the south-west and Wales. A popular bird, it was the subject of Ken Loach's famous 1969 film Kes, about a working-class boy in northern England who has a pet kestrel.
Its decline has not been thoroughly investigated, but the organiser of the new atlas, Dawn Balmer, suggested that agricultural intensification of its grassland habitat, especially in the west, might have led to a decline in its small mammal prey. The other two birds shown as declining, the lapwing and the yellow wagtail, are both farmland birds which are known to have been hit by intensification.
The BTO stresses that the data being released are strictly provisional. When the atlas is finally published, it is likely to show in graphic detail the success stories and failures of Britain's wild bird communities over the past two decades.
Some of the success stories are remarkable. Birds such as the red kite, the marsh harrier and the Dartford warbler have expanded enormously in range and number, while species such as the little egret have bred in Britain for the first time and now become established.
Other species, however, especially woodland birds such as the spotted flycatcher, the wood warbler and the lesser spotted woodpecker, have shown drastic declines.