The mystery of why British songbirds vanish during their annual migration south has always puzzled ornithologists. Hunters, bird nets or a loss of habitat in which to feed, have all been blamed for the disappearances. But it now seems an unexpected marine predator may also be picking off flocks...sharks.

Researchers at the Field Museum in Chicago were amazed to find the remains of sparrows, woodpeckers and doves in the stomachs of four in 10 of the tiger sharks they sampled in the Gulf of Mexico.

"Tiger sharks will see an easy meal and snatch it up, but I was surprised to learn that the sharks were eating songbirds I assumed that they'd be seabirds," said Dr Kevin Feldheim, a researcher at Chicago's Field Museum who led the DNA analysis which determined kinds of birds the sharks were eating.

Around 50 million birds set off from Britain each year in search of warmer weather as the winter approaches. But recent figures show there has been a significant decline in birds such as nightingales, turtle doves and wood warblers whose route takes them near the coast of Senegal, where 17 species of sharks live.