Tiger sharks feast when migratory birds fall out of the sky

It all started when a small tiger shark barfed up a bunch of feathers.

Marcus Drymon is a fisheries ecologist at Mississippi State University in Biloxi. He had been catching sharks as part of a long-term monitoring program in the north-central Gulf of Mexico. Typically, he and his colleagues would pull a shark out of the water for only about 90 seconds. That’s enough time to weigh and tag the fish before releasing it. But perhaps stressed by this experience, one tiger shark back in 2010 left its stomach contents on the boat’s deck.

“Being the science nerd I am, I scooped up all the feathers, all [that] tiger-shark barf, and put it in a bag,” Drymon says. Then he took it back to the lab.

Little did he know that those stomach contents would lead to a discovery about how young tiger sharks take advantage of bird migrations to get a free meal.

First, Drymon wanted to know which bird species the feathers had come from. So he reached out to an old friend. This was molecular ecologist Kevin Feldheim at the Field Museum in Chicago. Feldheim usually looks at shark DNA to study relationships between individual sharks. But he agreed to try to identify the feathers with DNA barcoding. This technique lets scientists identify a species based on a short strand of mitochondrial (My-toh-KON-dree-ul) DNA. The analysis showed that the feathers came from a songbird called the brown thresher.

Intrigued, Drymon searched through scientific journal articles. He found a handful that had reported tiger sharks eating terrestrial birds. And so he decided to investigate how common this bird eating was. He and his colleagues continued their monthly surveys of sharks from 2010 to 2018. Whenever possible, they collected the stomach contents of tiger sharks — by dissecting dead sharks or washing out the stomachs of live ones. In all, they got stomach contents from 105 tiger sharks and sent them to Feldheim to analyze.

Some samples “smelled pretty bad,” Feldheim recalls. Barely digested feathers weren’t too bad. He also found it easy to extract DNA from them. Other samples were harder to work with. “You could just take it out of the jar,” he says, “and it just smelled horrible. And those were usually very difficult to extract from.”

Free meals fall from the sky

Forty-one of the sharks (39 percent) contained bird remains from 11 species. These were eight songbirds (including barn swallows and house wrens) along with white-winged doves, yellow-bellied sapsuckers and the American coot. That last was the only water bird in the bunch. The researchers reported their findings online May 21 in Ecology.

“When he started sending me feathers, I just assumed they would all be seabirds — like gulls or pelicans or something like that,” Feldheim says.

The part of the Gulf where Drymon works is near the last spot migratory birds can land before they take off for Mexico in the fall. It’s also the first spot where many can land in spring after their marathon flight across the water. Drymon at first thought that the birds might be the ones that don’t quite make it to land in spring. But he looked at the timing of when the birds showed up in the sharks and could see that most of the birds were eaten in the fall.

“That,” he says, “was a puzzle.”

To make sense of it all, Drymon reached out to a Mississippi State colleague, Auriel Fournier. She is an ornithologist who is now director of the Forbes Biological Station in Havana, Ill. They consulted eBird, an online database of bird sightings. They also hashed out their data with other colleagues over lunches.

These birds must take off for their journey south and then get caught when the weather unexpectedly turns nasty, the researchers decided. And unlike ducks and other water birds, the species found in the sharks can’t keep themselves dry once they hit the water. So if they’re knocked out of the sky by something like a storm, they become shark bait, Drymon and his colleagues now propose.

“It made sense within what we know about bird migration,” Fournier says. “It’s just a piece of the puzzle that we didn’t know about.”

So, are these birds feeding other species in the Gulf, too?

No, Drymon says. “I’ve looked at the stomachs of so many other species of shark, so many other species of bony fish. And none of them eat these terrestrial birds — or marine birds, for that matter — the same way that the tiger sharks do.”

The birds’ fall migration does coincide with a peak in the young tiger-shark population in the north-central Gulf of Mexico. And about half the sharks that had birds in their bellies were in fact baby sharks. Drymon and his colleagues now think that mama sharks have figured out that the area is a good time and place to give birth. It gives their young access to an easy-to-obtain food source before they’ve fully honed their hunting skills. Says Drymon, “Why not take advantage of these deceased little songbirds that are apparently littering the surface of the ocean?”

That idea fits with tiger sharks’ will-eat-anything reputation. “I don’t like it necessarily when people say tiger sharks are the trash cans of the sea,” Drymon says. “That carries a kind of negative connotation.” He likens them to like teenage boys, gobbling up whatever food you put in front of them — even terrestrial birds that nothing else will apparently touch.

While Drymon, Feldheim and Fourier discovered something interesting, they all agree that one of the best parts of this study was their collaboration. “I saw something interesting, but it didn’t make sense to me,” Drymon says. He couldn’t figure it out on his own, so he brought in experts from in other fields. This let him do work outside his own field, “which is just a fun thing to do,” he says. Plus, “if you’re lucky, you get to collaborate with friends.”