Land Birds are Shark Prey

Researchers use gastric lavage to acquire a live tiger shark's stomach contents.

David Hay Jones

Tiger sharks (Galeocerdo cuvier) are known as the “garbage cans of the sea” because of their remarkably diverse diet. Their stomachs have been found to contain the remains of many types of animals, such as fish, squid, and sea turtles, and some manmade objects, such as old tires. A new long-term observational study found that tiger sharks in the north central Gulf of Mexico have consumed at least eleven species of birds since 2010—but the sharks were eating terrestrial birds, not marine birds.

From 2010 to 2018, as part of a shark monitoring program off the coasts of Mississippi and Alabama, researchers led by biologist Marcus Drymon of Mississippi State University sampled the stomach contents of 105 tiger sharks. Sampling typically involved a non-lethal technique called gastric lavage, in which water is poured down a tube into the shark’s mouth before upturning the shark, causing it to expel its stomach contents. This technique, along with gut analysis of dead sharks, produced bird remains—either whole or parts—in forty-one sharks. Using visual identification and DNA analysis conducted at the Field Museum in Chicago, Illinois, the researchers identified eleven bird species in the remains, a mix of passerines, near-passerines, and one terrestrial waterbird. They did not find any remains of seabirds.

All but one bird species was at least partially migratory, which led the researchers to investigate each species’ migration pattern across the Gulf of Mexico. Using bird observation data from the online citizen science database eBird, they found that the date of each bird-shark interaction occurred at or near the peak of that bird species’ migration through the area. Most of these bird-shark interactions occurred in the fall, when the birds were migrating south. Researchers think more frequent storms at this time of year may have forced birds to the water’s surface. Even without the presence of scavenging tiger sharks, this is likely a fatal move for a terrestrial bird, which, once on the water, cannot rest or take flight.

With over two billion birds taking this migratory path each season, downed terrestrial birds are likely a common source of food for tiger sharks in the north central Gulf. Future studies may examine whether tiger sharks in other areas also consume terrestrial birds and how these interactions occur. (The Scientific Naturalist)