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.
Tiger sharks are known as the ‘dustbins of the ocean’ because they will eat cows and deer which fall into the water and even indigestible man-made objects such as books, tires, number plates and jewellery.
They are also known to seek out the fledging grounds of seabirds like albatross, but it’s the first time they have been found to eat songbirds.
The researchers believe the birds were on their annual migration and probably landed on the sea to rest, or became fatigued and fell into the water.
For the study a team from Mississippi State University caught 105 three-foot long juvenile tiger sharks, then pumped their stomachs before returning them unharmed to the sea.
They found that 41 had partially digested bird remains in their stomachs which were sent to the Field Museum for DNA analysis.
"None of them were seagulls, pelicans, cormorants, or any kind of marine bird," said lead author Dr Marcus Drymon of Mississippi State University.
"They were all terrestrial birds, the kinds that might live in your backyard.”
The study had coincided with peak migration time when songbirds were seen off the US coastline heading south.
“The tiger sharks scavenge on songbirds that have trouble flying over the ocean,” added Dr Feldheim.
“The terrestrial birds might make more attractive prey than seabirds because the seabirds can handle themselves better in and around the water than the songbirds.”
Martin Fowlie, of the RSPB, said the study may give another clue as to what was killing off birds during their annual migration.
“Migratory birds face numerous dangers on their amazing journeys; from the loss of places to feed, to hunters with guns and nets,” he said. “This new study adds an unexpected new one, sharks!”
The British Trust for Ornithology (BTO) said they suspected the birds found inside the sharks were the ones that struggled to complete the migration and may have fallen into the water after becoming too tired to go on.
The research was published in the journal Ecology.