By Richard Gray, Science Correspondent
05 Sep 2010 8:15AM BSTOrnithologists have found that species including the turtle dove, willow warbler, tree pipit and redstart are struggling to find enough food in the weeks before they set off in the spring to fly to the UK.
The scientists believe that years of poor rainfall in sub-Saharan Africa have reduced supplies of the seeds, fruits and insects which the birds rely on to build up vital energy supplies.
The finding could explain a steep decline which has led to many migratory birds being listed as threatened species in the past decade.
Conservationists had been searching for an explanation, with some blaming farming practices in the UK including the removal of hedgerows and the widespread use of pesticides.
Researchers at the British Trust for Ornithology (BTO), who will present their results at the annual conference of the British Ecological Society on Tuesday, looked at populations of 16 bird species over the past 40 years, during which time some have declined by as much as 85%.
They found that migratory bird populations in the UK fluctuate annually by as much as one-third in some species, depending on the wet-season rainfall levels in Africa and levels of vegetation cover, which were measured from satellite images.
Migratory birds tend to arrive in Britain around April. After nesting and breeding, they depart around October. Many overwinter in west Africa and the Sahel region on the southern fringe of the Sahara desert.
The birds arrive in Africa at the end of the rainy season when there is lots of food available, but over the following months conditions get progressively drier. It is the amount of rainfall in the rainy season that determines how long the resources last. …
The whitethroat and sedge warbler, both threatened species which spend winters in the Sahel, have declined by 67% and 41% respectively. The redstart, which migrates to west Africa, has declined by 14%.
Even species which spend their winters further south in Africa where rainfall is more consistent, such as the swallow and the willow warbler, can still be affected by rainfall levels in the Sahel. …
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