During a 2010 shark tagging project in the Gulf of Mexico, researchers inadvertently captured a small tiger shark that regurgitated feathers. Not just any feathers, mind you. These were feathers belonging to a brown thrasher, a land-based backyard songbird not particularly known for its seafaring ways. The event sparked the interest of the research team and launched a nearly decade-long study exploring why tiger sharks were chowing down on land-based birds. Their results are now published in the journal Ecology.

But first, a quick lesson on Galeocerdo cuvier. These voracious predators are notorious for eating pretty much everything – turtles, birds, and even garbage like tires, nails, and license plates. Their wide variety of prey is an adaptation for survival. Sharks reaching lengths of up to 3 meters (5 feet) require a lot of energy, after all. We’ve known for some time that tiger sharks make massive treks to find their food, such as albatross fledglings in Hawaii and nesting green turtles in Australia. But woodpeckers, sparrows, and doves? Yeah, that’s a new one.  

"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," said study co-author Kevin Feldheim of the Field Museum, who led the DNA analysis on identifying the birds' remains, in a statement. "It was one of the coolest projects I've been associated with using DNA to tell a story."

Between 2010 and 2018, the team, led by Marcus Drymon of Mississippi State University, opportunistically pumped the stomachs of juvenile tiger sharks – at less than a meter long (3 feet) they could wrestle them onboard – that they caught during tagging and other research projects. The young sharks were unharmed and were released back into the sea afterwards. Of 105 sharks studied, 41 contained partially digested bird remains. The team then cataloged small pieces of these remains and used chemicals to break them down into basic molecular components so that they could examine the DNA sequences and compare against analytics collected by international bird database eBird.

"None of them were seagulls, pelicans, cormorants, or any kind of marine bird," said Drymon. "They were all terrestrial birds – the kinds that might live in your backyard.”

In all, 11 separate bird species were identified and in every instance, the timing of the young tiger sharks eating land birds coincided with peak sightings of that species off the coast. 

"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," said Feldheim, adding that terrestrial birds might make for easier prey than their seafaring cousins because the seabirds can handle themselves better in and around the water.

The researchers suggest this means the sharks' consumption of the land-based birds is tied to predictable seasonal migrations, and that they are purposely coinciding their foraging with seasonal peaks in available food. The fact that it was mainly juvenile sharks that ate the birds also suggests that scavenging on easily accessible food sources that are predictable by season may be advantageous for young sharks before they have the skills to hunt as adults.

This the first evidence scientists have of tiger sharks eating backyard songbirds and it could lead to better understanding and conservation of the near-threatened species, the researchers say.