By Rebecca Morelle
BBC News science reporter
Migratory birds have suffered a dramatic decline in numbers, according to a study.
The birds that winter in Africa are most affected
Species that migrate thousands of miles from Africa to the UK have been the worst hit over the last 30 years.
The researchers say the cause of the decline remains a "mystery", but could be linked to climate change, habitat destruction or pesticide use.
Writing in the journal Biological Conservation, they warn the losses may indicate wider environmental damage.
The Royal Society for the Protection of Birds (RSPB) and BirdLife International study analysed population trends of European breeding birds, including non-migratory birds and those that migrate both short and long distances.
The data spanned three decades, from 1970 to 2000.
"We found that long distant migrants - the ones that go right across the Sahara, like the swallows, flycatchers and warblers - have shown a fairly consistent pattern of decline," said Dr Paul Donald, an author on the paper from the RSPB.
Those that winter in Africa, he said, seem to be the most affected.
The study also compared the long-distant migratory birds with closely related non-migratory birds, but again found in almost every case that the migratory birds faired worse.
The roller has suffered dramatic losses
Fifty-four percent of the 121 long-distant migratory birds studied suffered plummeting numbers or had even become extinct since 1970.
The roll-call of declining species is long.
"Some fairly iconic species have declined enormously in Europe. There is a very beautiful blue and purple bird called the roller - the population of that bird is crashing all over Eastern Europe," Dr Donald told the BBC News website.
"In the UK, other species that have declined enormously are spotted flycatchers, pied flycatchers, wheatears, wood warblers and tree pipits."
The exact reason for the birds' decline, according to the authors, is a "mystery". But several theories to explain the losses have been put forward, and will now be investigated.
One explanation is tied to the changing conditions in Africa, where the birds winter.
"We know that agriculture has spread; we know there has been a long-term drought in the Sahel; and we know huge amounts of pesticides are used to control locust outbreaks," said Dr Donald.
The wryneck no longer breeds in the UK
The swelling size of the Sahara may also be hampering the birds. Migrating birds face longer and longer non-stop flights across the desert.
Climate change has been highlighted as a potential culprit. Warmer springs in Europe are causing some insects to hatch earlier in the year, which means by the time the migratory birds arrive to breed and raise their young they may have missed their much-needed food-source.
"Migrants make up a high proportion of our species of birds, so this is a big conservation issue," said Dr Donald.
"But if you think that these are birds that cover vast areas of the Earth's land-surface - this consistent pattern of decline is indicative that there are some pretty severe environmental changes going on somewhere which might also have an impact on humans."
The authors conclude that urgent action is needed to uncover the cause of the decline.
"There is something about being a migrant that counts against them," said Dr Donald.
"These birds have been slipping away from under our noses for 30 years, and we've never has really noticed it before."