Why So Many Sharks Have Bird Feathers in Their Bellies

Spoiler: Migration is hard.

Feathers from the stomach of a tiger shark
Feathers from the stomach of a tiger shark (Marcus Drymon)

Marcus Drymon wasn’t expecting a baby shark to barf up a ball of feathers onto his boat.

The shark’s presence wasn’t the weird bit: Drymon and his team of fisheries ecologists regularly assess fish populations along the coasts of Mississippi and Alabama, and every year, they’ll catch, weigh, tag, and release thousands of sharks. In 2010, they were doing just that for the meter-long tiger shark when it coughed up the feathers. “Being an ecologist, I scooped them up and took them back to the lab,” Drymon says.

He passed the feathers to Kevin Feldheim, a molecular biologist at the Field Museum, who analyzed the DNA within them to work out what species they belonged to. The answer: a brown thrasher, a thrush-like songbird that lives in forests. What on Earth was it doing in the belly of an oceanic apex predator?

“I had expected a laughing gull or a brown pelican,” Drymon says. “The brown thrasher was the last bird I would have expected.”

In fairness, a brown thrasher is hardly the weirdest thing to end up in a tiger shark. This species is notorious for eating pretty much anything. Aside from the remains of dolphins, dugongs, sea turtles, and sea snakes, scientists have found tires, license plates, a drum, unexploded munitions, and an entire chicken coop inside tiger-shark stomachs. And when Drymon dug through some old papers, he found a few decades-old records of tiger sharks eating land-based birds. Thinking about his brown thrasher, he wondered, “Was this just a one-off anecdote, or is there some sort of a pattern?”

For the next eight years, during his annual surveys, Drymon checked any tiger sharks he caught for feathers. The method is simple: Put a wide tube in the shark’s mouth, thread a hose down the tube, turn the shark upside down, and catch whatever comes out. Amazingly, they found bird remains in 41 out of 105 sharks—39 percent!

In most cases, the feathers were so degraded, and swamped by DNA from the sharks and their other prey, that the team couldn’t identify them. But they managed to do so for 11 samples. Barn swallow, house wren, common yellowthroat, yellow-bellied sapsucker, American coot ... all land-based, with not a single seabird among them. In a few cases, the birds were only partially digested, “and the DNA barcoding wasn’t necessary,” Drymon says. “We could identify them from a field guide.”

I’m no expert, but I think it’s probably a bird. Credit: Marcus Drymon

“We’re seeing these interactions every single year,” he adds. “All of a sudden, it’s not just a gee-whiz thing. There’s something driving these interactions, in a predictable way.”

He was stumped until he had lunch with Auriel Fournier, a bird ecologist who works in the same building. She told him that large flocks of birds migrate over the Gulf of Mexico, and presumably some of them fall into the sea because of bad weather, exhaustion, or some other unlucky event. It wasn’t surprising that a shark should eat them. But Fournier was surprised at how often it was happening.

She consulted eBird—a database run by Cornell University, which collects sightings from bird-watchers worldwide. Pulling data from the Mississippi and Alabama coasts, Fournier showed that the dates at which Drymon pulled feathers from his tiger sharks coincided almost exactly with the peak sighting times for almost all of the 11 species.

Nature’s migrations are so epic that it is easy to forget how treacherous they are, and that many travelers simply never get to their destination. Consider the migrating birds that travel southward over the Gulf of Mexico from later summer to fall: They have to make the crossing without stopping. They don’t have the water-repelling oils that seabirds use to coat their feathers, so if they land in the sea, “they suck up all that water, and become so soaked that they can’t get up again,” says Fournier. “If a bird has to make that landing, it’s probably done for.”

No one knows how often this happens. But with an estimated 2 billion birds crossing the gulf every year, a low chance of death still translates into a lot of floating bodies—and a lot of meals for scavenging sharks.

Drymon says that the waters near Mississippi and Alabama are rich in very young tiger sharks, and he only ever found bird remains in these small individuals. “This scavenging on easily accessible prey could represent a way for these young sharks to feed themselves before they’ve learned adult hunting behavior,” he says. “It’s almost like baby food.”

This could also help to explain why tiger sharks, unlike many other species, don’t deposit their young in sheltered estuaries and mangroves, and instead give birth in more open waters. Those areas might offer less protection, but if they predictably receive feathery manna from the heavens, that would benefit the baby sharks. And so two worlds—forests and oceans, songbirds and sharks—briefly collide. The same could be said for the researchers involved in this study. “It’s a fun way of bringing together these two parts of the biology world that don’t talk to each other very often,” Fournier says.

“A fisheries ecologist, a molecular biologist, and a bird ecologist ... it’s like the start of a bad joke,” Drymon adds.