Animals

Giant tiger sharks eat backyard birds, surprising study reveals

This is the first scientific confirmation that the voracious predators regularly eat land-based birds.Wednesday, May 22, 2019

By Jake Buehler
A tiger shark swims off the Bahamas. The predators are consummate scavengers, with excellent senses of sight and smell.

Tiger sharks are among the most imposing predators of tropical seas. Reaching lengths of 15 feet or more, the beastly fish also have voracious appetites, devouring everything from sea turtles to rubber tyres.

Now, new research suggests that baby tiger sharks have an even weirder item on their menu: common backyard birds.

Study leader Marcus Drymon stumbled upon the discovery in 2010, when he caught a small tiger shark as part of an ongoing research project in the Gulf of Mexico.

“We brought it onto the boat to get ready to measure it, weigh it, tag it, and release it, and it barfed up this big [mass] of feathers,” says Drymon, a marine fisheries researcher at Mississippi State University.

A marsh wren sings in beachgrasses on New Jersey's Money Island.

“Being a scientist, of course, I scooped them all up and took them back to the lab to analyse.” (Read about what it's like to go face-to-face with a tiger shark.)

It turned out the feathers belonged to a brown thrasher, not one of the many seabirds—such as gulls, pelicans, or cormorants—that tiger sharks regularly eat.

Other scientists had reported tiger sharks eating terrestrial birds before, but these were anecdotal, one-off occurrences. The new study, published May 21 in the journal Ecology, is the first confirmed evidence of tiger sharks regularly eating terrestrial birds.

Chasing sharks

After that initial discovery, Drymon and colleagues decided to investigate tiger shark diets as part of their project.

When the scientists caught tiger sharks that were small enough to wrestle on board and handle, they’d perform a “gastric lavage” by placing a PVC pipe in the shark’s mouth, putting a hose down the pipe, and rinsing the stomach contents out into a sieve to be catalogued. The shark—often a newborn only three feet long—was released afterward, unharmed. 

Over eight years, out of 105 sharks studied, 41 had partially digested bird remains in their guts. When the team identified the remains using DNA analysis, they found that they all came from 11 species of North American land birds.

A swamp swallow forages at Blackwater National Wildlife Refuge in Maryland.

Birds on the move

Bizarrely, most of them, like swamp sparrows and marsh wrens, were passerines—the chirping songbirds that you might see at your bird feeder.

But how did scores of avian landlubbers find their way to such salty doom? Drymon and his colleagues used data from the citizen science project eBird to find the seasonal timing of when the birds in the study were most abundant off Mississippi and Alabama. Remarkably, each species showed up in shark bellies right around the time its local prevalence peaked. (Learn more about tiger sharks.)

A shark threatens a fledgling albatross on French Frigate Shoals in Hawaii.

“It's a strong suggestion that this phenomenon is linked to [songbird] migration,” says Drymon.

Since most of the songbirds were eaten in the fall, Drymon thinks the birds may be getting struck down to the ocean’s surface by severe and unpredictable autumnal storms as they leave for southern latitudes, making them easy prey for the sharks.

It also seems as though the sharks have noticed the seasonal glut of feathered calories, since Drymon’s team found juvenile tiger shark abundance in the area is three times higher between August and November. Such food-motivated aggregations of tiger sharks are seen elsewhere, such as at French Frigate Shoals in Hawaii, where the predators feed on fledgling albatross. (See gorgeous photos of sharks.)

Baby food?

“But what makes our situation even more interesting is that the vast majority of these individuals are newborn tiger sharks,” says Drymon.

It’s even possible that tiger shark mothers birth their pups in the northern Gulf because the young can capitalise on the seasonal songbird scavenging opportunities.

Samantha Munroe, a marine biologist at the University of Adelaide in Australia, says by email that the study “helps change our perception about the connectivity between land and marine species.” (Read about threats to songbirds.)

It also “provides a great jumping-off point for a myriad of other interesting research questions,” says Munroe, who was not involved with the study.

For instance, are songbirds more nutritious than other prey? “Future studies assessing the energy or nutritional value of terrestrial birds as compared to other types of prey," she says, "would also help clarify how important songbirds are to tiger shark diet and survival.”

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