Tiger sharks eat songbirds
Study of stomach contents finds no seagulls, plenty of woodpeckers. Tanya Loos reports.
New research reveals a surprising component of young tiger shark diets: land birds.
Tiger sharks (Galocerdo cuvier) are known for their wide and varied food intake, including live prey such as fish, sea turtles, marine mammals, and sea birds. They also scavenge on whale carcasses, and even the odd car tyre.
Researchers have long suspected that terrestrial land birds are also eaten, but until now no study had assessed the mechanism of this unusual trophic interaction.
“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,” says Kevin Feldheim, a researcher at Chicago’s Field Museum in the US, and a co-author of the study.
In 2010, Marcus Drymon from Mississippi State University, US, was carrying out routine population monitoring when a small tiger shark regurgitated feathers prior to being tagged and released.
Subsequent DNA analysis revealed the feathers belonged to a brown thrasher (Toxostoma rufum), a common backyard bird in Canada and south-eastern US.
Intrigued by this discovery, Drymon and fellow researchers added another element to the monthly surveys: any young tiger sharks caught had their stomach contents pumped, before being released unharmed. Partially digested feathers are difficult to identify, so bird remains were sent to the Field Museum’s laboratory for analysis.
Of the 105 sharks examined, 41 (39%) contained bird remains. The DNA analysis facilitated conclusive identification of 11 species.
And what that revealed was weird.
“None of them were seagulls, pelicans, cormorants, or any kind of marine bird,” Drymon says. “They were all terrestrial birds.”
They included types found in many American backyard, including songbirds such as the house wren (Troglodytes aedon) and eastern kingbird (Tyrannus turannus), and even a species of woodpecker, the yellow-belled sapsucker (Sphyrapicus varius).
Most of the birds were eaten by tiger sharks in the autumn, although some remains showed up in young sharks during spring. The team examined a large online database of bird sightings known as eBird to assess the movements of terrestrial songbirds at these times of year.
The twice-yearly migration of songbirds across the Gulf of Mexico involves in excess of two billion birds each season. The team found that these migration periods coincide with a peak of young sharks in the Gulf.
“In every instance, the timing of the tiger shark eating the bird coincided with the peak sighting for that species of bird off our coast,” says Drymon.
Feldheim adds: “The tiger sharks scavenge on songbirds that have trouble flying over the ocean. During migration, they’re already worn out, and then they get tired or fall into the ocean during a storm.”
The findings suggest that exhausted songbirds are an easily accessible and seasonally predictable pulse of nutrients for the young sharks. The inclusion of terrestrial birds in their diet is an unusual trophic interaction between land and sea ecosystems, because unlike seabirds and guano, the energy exchange is reversed from land to sea.
The study also illustrates the benefit of access to an extensive DNA database.
“It shows us how much more we can still learn about sharks in general and what DNA can tell us that observation can’t,” says Feldheim. “It was one of the coolest projects I’ve been associated with using DNA to tell a story.”
The research was published in the journal Ecology.