For migrating songbirds, ‘baby shark’ is more than just an annoying tune

During a routine shark survey in the northern Gulf of Mexico in 2010, ecologist Marcus Drymon caught a small tiger shark. At first glance, there was nothing unusual about it. But when his team brought the fish to their boat to measure, weight and tag it, before releasing it, the shark regurgitated a bolus of feathers.

“Intrigued, I scooped them up and brought them back to the lab,” Drymon, an assistant extension professor at Mississippi State University and marine fisheries specialist with the Mississippi-Alabama Sea Grant, told Mongabay.

He handed over the feathers to Kevin Feldheim, a molecular biologist at Chicago’s Field Museum, who analyzed DNA from the feathers to trace it back to the bird they had come from. The feathers, it turned out, weren’t from a marine bird, but rather belonged to a brown thrasher (Toxostoma rufum), a small reddish-brown songbird that’s fairly widespread across the eastern and central United States. This was unexpected.

Over the next eight years, Drymon and his colleagues began to opportunistically check the stomach contents of tiger sharks (Galeocerdo cuvier) they caught during their surveys for bird feathers, beaks and feet. And nearly every year, the team came across tiger sharks that had bird remains inside them, the researchers report in a new study published in the journal Ecology.

Of the 105 sharks they examined, 41 (or 39 percent) had some bird beaks, feet or feathers in them. Not all remains could be analyzed further, but the researchers managed to trace remains from 13 tiger shark-bird interactions to 11 species of birds, most of them land-dwelling songbirds like the barn swallow (Hirundo rustica), house wren (Troglodytes aedon), common yellowthroat (Geothlypis trichas) and marsh wren (Cistothorus palustris). There were no marine bird remains in these sharks at all.

“It was certainly surprising to me!” Drymon said in an email. “If a tiger shark is going to have access to a bird (either alive, or a bird carcass), I would have assumed it would be a marine bird like a pelican, gull, or cormorant.”

It isn’t that tiger sharks don’t eat marine birds; they do. Tiger sharks are known to travel to remote Hawaiian islands specifically to feed on fledgling albatross, for example. In South Africa, too, they’re known to eat a variety of marine birds, Drymon said. So why are tiger sharks eating land-dwelling songbirds?

To find out, Drymon and his colleagues turned to eBird, a massive citizen science project in which birdwatchers add their bird sightings from around the world. They found that the months during which they encountered tiger sharks with birds in their guts coincided with the peak timings for coastal bird sightings for the 11 species, suggesting that the shark-bird interactions could be linked to the annual migration of those terrestrial birds.

In the spring, many migratory songbirds leave their wintering grounds in Central and South America to fly north, crossing the Gulf of Mexico to reach their first stopover, the coasts of Mississippi and Alabama. In the fall, the same areas become their final stopover before they begin their crossing of the Gulf of Mexico again. The researchers say they initially thought there would be more tiger sharks eating migrating birds in the spring, when the birds would be fatigued by their long journey across the Gulf of Mexico and drop into the water. To their surprise, most of the interactions they documented occurred in the fall months.

“My original line of logic was as follows: if tiger sharks are eating migratory birds, perhaps it’s because these birds don’t have the energy to make it across the Gulf of Mexico,” Drymon said. “But this wasn’t the case — most interactions took place in the fall, which is when these birds would just be starting their long journey across the Gulf of Mexico, and presumably well rested and well fed.”

Drymon and his colleagues speculate that unpredictable storms and weather events could be forcing the migratory birds to the water as they begin to take off for their marathon journey southward. Unlike marine birds that are better at handling water, land birds become easy prey. But confirming if weather is actually responsible would require more data. “I know that’s a typical answer from a scientist, but it’s true,” Drymon said. “Relating the timing of storms to feeding event will require fine-scale data that can only be collected with high-tech equipment like satellite tags.”

Surprisingly, half of the tiger sharks that had songbirds in their bellies were babies. Drymon said he suspects that for these baby sharks, which haven’t yet learned to forage, scavenging on fallen birds might be an attractive food source. And mother sharks could be actively selecting such areas to give birth.

“If adult female tiger sharks are indeed ‘choosing’ areas in the northern Gulf of Mexico to birth their brood, then these areas may warrant special consideration, both for the adult females and the newborn sharks,” Drymon said.

Citation:

Drymon, J. M., Feldheim, K., Fournier, A. M., Seubert, E. A., Jefferson, A. E., Kroetz, A. M., & Powers, S. P. (2019). Tiger sharks eat songbirds: Scavenging a windfall of nutrients from the sky. Ecology. doi:10.1002/ecy.2728