Ornithologists 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.
Dr Nancy Ockendon, a research ecologist at the BTO, said: "If it has been a dry year, by the time the spring comes and the birds are thinking about building up the reserves they need to fly back over the Sahara, there is much less available to them and this affects the number of birds making it back to Britain.
"They are either just not surviving the journey or it takes them too long to get the reserves they need and they are too late to leave. We don't know the mechanisms yet and our work is still ongoing."
The tree pipit, a rare bird which spends the winter in west Africa, has experienced an 85% drop in numbers since 1966. Turtle dove numbers have fallen by the same amount.
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.
The researchers believe this may be because they stop off in the region on their way back to Britain.
Dr Ockendon added: "It has been increasingly observed over the last decade or so that the migrants that breed in Britain and winter in Africa are showing really dramatic declines.
"Quite a lot of them are red-listed species and of conservation concern.
"The changes in rainfall in Africa could be leaving the birds less able to cope with growing pressures back here in the UK as well. – when it is a bad year in Africa they can't recover so quickly because their habitat has been degraded.
"There is less slack in the system now."
Severe droughts in Africa during the 1960s were observed to bring short-term reductions in the number of migratory birds arriving in the UK, but this latest research provides evidence that changes in rainfall have have a longer-lasting impact.
Bird experts now fear that as climate change and changing land use in Africa increasingly cause water shortages, numbers of migrating birds will continue to suffer.
A spokesman for the RSPB said: "With summer-visiting birds we are facing a crisis in terms of the sharp rate at which these birds are disappearing.
"The cause behind this is very complicated and difficult to disentangle, but this research suggests the changing water regimes in western central Africa are playing a significant role
"There is growing concern about how the significant variability in land use and water availability in Africa is impacting on summer visitors to the UK and Europe."
Tree Pipit
