Science

Exhausted Birds Become Lunch Meat for Tiger Sharks in the Gulf of Mexico

New research shows that these apex predators wait for food to fall out of the sky during migration.

Like teenage boys, tiger sharks gobble up virtually everything in sight. From sea turtles and license plates to an entire chicken coop and a horse head: You name it, it’s been found in a tiger shark’s stomach.

Even so, Marcus Drymon, a marine fisheries ecologist at Mississippi State University who studies tiger sharks, didn’t expect one of his study subjects to stress vomit feathers onto the deck of his boat in 2010. Further inspection revealed that the plumes belonged to a Brown Thrasher, a slender songbird that frequents gardens and thickets in the eastern United States.

Intrigued, Drymon checked the scientific literature for clues. He found a few documented cases of the apex predators snacking on land birds, starting with a Yellow-billed Cuckoo that was devoured off Florida’s Gulf Coast in 1961. But no one knew whether bite-sized morsels like warblers, wrens, and sparrows were a regular part of the tiger shark diet, or whether they were anomalies like the horse head and the chicken coop.

Drymon has been looking into the matter ever since—and appears to have solved the mystery in part. His findings, published in Ecology this week, suggest that whenever storms kill migrating birds over the Gulf of Mexico, tiger sharks receive a “windfall of nutrients from the sky, he says. “It’s an example of how two completely disparate food webs can interact in a way we wouldn’t have predicted.

Roughly two billion birds cross the Gulf of Mexico each spring, and even more likely fly back over it in the fall. Some are inevitably blown into the sea by storms, or crash onto the waves while facing headwinds, fog, or disorienting lights from oil and gas platforms. “Once they hit the water, they’re pretty much done,” says co-author Auriel Fournier, a Mississippi State ornithologist, who points out that even the healthiest land birds typically can’t get airborne when their feathers are soaked.

That’s when the sharks pounce, snapping up already-doomed individuals that are at or just below the surface of the waves. Drymon, who hopes to one day witness this phenomenon in action, suspects that tiger sharks prey on passerines in the same spectacular fashion they devour albatross fledglings in the Hawaiian islands: They lift their upper jaw out of the water and drag the bird down in one swift move, he says.

Interestingly, no other shark species seems to be taking advantage of this easy, migratory food source; as far as we know, only tiger sharks have developed a taste for marine and terrestrial birds, Drymon says. He does his research off the coast of Alabama and Mississippi, using a mile-long line with 100 baited hooks to haul in large fishy predators. He and his team then wrestle each catch onto the boat—no easy task, considering tiger sharks grow up to 15 feet long and are quick to bite—and pump its stomach with a basic, clear tube. In most cases, the animal is released unharmed.

Marcus Drymon (right) and a colleague gather stomach contents from a young tiger shark in the Gulf of Mexico. Photo: David Hay Jones

Between 2010 and 2018, Drymon examined the stomach contents of 105 tiger sharks and found bird remains in 41 of them. Most were too masticated to identify by sight, so Drymon shipped them to the Field Museum in Chicago, where researchers used DNA barcoding tests to determine their origins.

Many of the samples were either too putrid or tainted with other creatures’ DNA for successful genetic analysis. Nonetheless, Kevin Feldheim, lab manager at the Field Museum’s Pritzker Laboratory, and Fournier were able to ID 13 of the 41 birds.

“When [Marcus] told me they were from tiger shark stomachs I assumed they would all be from marine birds like pelicans or gulls or something like that,” Feldheim says. Instead, 12 of the 13 remains were of small land birds, including a Barn Swallow, a Marsh Wren, an Eastern Kingbird, a Common Yellowthroat, and a pair of Yellow-bellied Sapsuckers. Even the one water bird, an American Coot, registered as an oddity for the deep Gulf waters.

Data from eBird shows that these species were generally being eaten during their peak migration time for the region, leading the team to believe that tiger sharks, like birders, actively search for fallout. In fact, 11 of the 13 digested specimens had been flying through in autumn, when the northern Gulf becomes a known hotspot for young tiger sharks. Drymon has a hunch that female tiger sharks choose to give birth in the area so that their babies can dine on the birds that drop down like manna from heaven. “It’s something easy for them to eat before they gain the hunting skills of an adult,” he says—not all too different from a high schooler cooking a frozen pizza. Hey, they have to get their calories somewhere.

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