Chris Mead, of the British Trust for Ornithology, said that a few species were now known to be particularly vulnerable - such as barn owls, which huntalong long roads. Of 48 barn-owls which he had ringed recently, nine were recovered dead after being hit.
But Mr Mead thought collisions were also a significant cause of mortality in dozens of other species and a likely factor in the decline of the house sparrow. It may be one of Britain's most common birds but its numbers have fallen noticeably in urban and rural areas in recent decades.
"My own guess is that between 3 and 20 per cent of vulnerable British birds are killed each year on the roads," he said. About 100 species were vulnerable, he added - birds which flit between hedgerows, or which chase each other furiously in territorial disputes.
"It's the busier rural roads such as two-lane A roads which are the real killers," he said. "Motorways are probably much safer because the amount of noise and movement and their width keeps many small birds from attempting to cross them."
Mr Mead said the increase in average speeds of cars was adding to the cull. Young, inexperienced birds were specially vulnerable. "We're cruising at over the crucial level of 45-50mph - faster than that and there is little chance of them being able to react," he said. He believes that one reason why the magpie population has soared is because collisions provide them with carrion when food would otherwise be scarce.
But vehicle strikes are not the only danger that traffic poses to birdlife. From the Netherlands comes research which shows that willow warblers more than a mile from motorways are affected by the noise which "makes it difficult for them to hear each other, which they need to do in defending territory and attracting mates".
As well as new roads destroying habitat, they are also preventing individuals of a species from dispersing. Penny Anderson, an environmental consultant, cited other research from the Netherlands which examined the genetics of a population of frogs confined to a triangle of land by roads and railway. Over the decades their genetic diversity shrank because the limited population could only inbreed.