Quirks & Quarks

Flying food for fish? Tiger sharks are somehow eating songbirds

Migrating birds who fall into the ocean become fast food for baby tiger sharks

Migrating birds who fall into the ocean become fast food for baby tiger sharks

Scientists gathering stomach contents from a baby tiger shark (Marcus Drymon)
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Researchers studying the diets of juvenile sharks in the Caribbean were shocked to discover that the songbirds we normally associate with forests and meadows may be a significant part of the baby shark diet.

Tiger sharks have a reputation as the garbage cans of the ocean and are known to gulp down metal, tires and plastic bottles, in addition to their regular diet of fish, sea snakes and sea turtles. 

But it was a huge surprise to discover that they were eating things like house wrens, barn swallows and eastern meadowlarks, biologist Kevin Feldheim told Quirks & Quarks host Bob McDonald.

Feldheim and his colleagues analyzed the stomach contents of more than 100 tiger sharks from the Gulf of Mexico at various times of the year, and found the remains of 10 species of terrestrial birds. All in all, about 40 per cent of the sharks had been eating songbirds — mostly the youngest sharks.

A serendipitous discovery

Back in 2010, Marcus Drymon, the lead author of the study, was conducting general surveys of tiger sharks in the Gulf Mexico to better understand them.

One of the baby tiger sharks he was examining threw up when it came on board the research vessel, and Drymon noticed a feather in the spillage.

He took the feather to an ornithologist who identified it as coming from a songbird called a brown thrasher. That unusual event aroused his curiosity and motivated him to begin a study looking at the diet of the sharks.

Bird feathers retrieved from a baby shark's stomach (Marcus Drymon)

This began a study that involved capturing young sharks in the Gulf of Mexico and, essentially, pumping their stomachs. Drymon used a method called gastric lavage, in which a PVC pipe is inserted into the shark's mouth.  A hose is pushed down and water washes out the stomach contents which is collected for analysis.

Over several years, the team looked at 105 sharks and they were surprised to find that 41 had bird remains in their stomachs. Some of the birds were easy to identify by just looking at their feathers. He shipped the more difficult ones to Feldheim, the A. Watson Armour III manager of the Pritzker Laboratory for Molecular Systematics and Evolution at the Field Museum in Chicago.  

What are songbirds doing in shark stomachs?

Feldheim separated the feathers from the rest of the stomach contents and rinsed them to get rid of any contaminants. He then extracted DNA sequences from the feather shafts and compared them to a large database of animals to identify the bird species.

He found a variety of common songbirds like barn swallow, eastern kingbird, common yellowthroat, and house wren but noted that no marine birds were found. Tiger sharks had previously been observed eating albatross chicks in Hawaii.

Researchers think tiger sharks might prefer land birds since they're easier to catch than sea birds that are well adapted to water. This might be especially true for baby tiger sharks that are too young to be adept hunters.

An example of bird remains recovered during the study (Marcus Drymon)

Using the citizen science bird database eBird, researchers found that the most frequent findings of songbirds in shark stomachs coincided with the peak sightings for a species of bird off of the coast, confirming that most of the tiger shark-songbird interactions occurred during migration.

Large flocks of terrestrial birds fly over the Gulf of Mexico in the fall and spring. Feldheim speculates that the weaker birds may fall into the water when exhausted, or when a powerful storm hurls them into the ocean.  

"Marine birds are used to being around the water," explained Feldheim. "When they get tired of flying, they can land on water, and easily take off. But freshwater birds can't do that, and as a result, might end up as food for tiger sharks."

Implications for tiger shark conservation

Shark predation is not likely to be a major risk to songbird populations, explained Feldheim.

"Once these birds are blown into the water, they're done. The tiger sharks are just there to pick up what's going to be dead anyway."

However, it's an open question whether the continuing decline in songbird numbers as a result of habitat loss, among other factors, could have an impact on the sharks. While this study confirms that songbirds are part of the shark diet, it's not yet clear if they're an important part, and whether a decline in songbird numbers would have any impact on juvenile sharks.

"These terrestrial birds could be an important food source for young tiger sharks until they develop hunting skills," said Feldheim.

Going forward, researchers are planning to continue their shark diet surveys to get a more complete picture of how often they feed on terrestrial birds throughout the year.

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