Tiger sharks feast on migratory birds that fall out of the sky
Science News said:It all started when a small tiger shark barfed up a bunch of feathers.
Marcus Drymon, a fisheries ecologist at Mississippi State University in Biloxi, had been catching sharks as part of a long-term shark monitoring program in the north-central Gulf of Mexico. Typically, a shark spent only about 90 seconds out of the water, enough time for scientists to weigh and tag it before releasing it. But one day in 2010, a tiger shark, perhaps stressed by the experience, left its stomach contents on the boat’s deck.
“Being the science nerd I am, I scooped up all the feathers, all [that] tiger shark barf, and put it in a bag and took it back to the lab,” Drymon says.
Little did he know that those stomach contents would lead to a discovery about how young tiger sharks take advantage of bird migrations to get a free meal.
First, Drymon wanted to know what bird species the feathers had come from. So he reached out to an old friend, molecular ecologist Kevin Feldheim of the Field Museum in Chicago. Feldheim usually looks at shark DNA to study relationships between individual sharks. But he agreed to try to identify the feathers with DNA barcoding, a technique that lets scientists identify a species based on a short strand of mitochondrial DNA. The analysis showed that the feathers came from a brown thresher, a type of terrestrial songbird.
Intrigued, Drymon searched the scientific literature, finding a handful of reports of tiger sharks eating terrestrial birds, and he decided to investigate how prevalent these meals were. During monthly surveys from 2010 to 2018, he and his colleagues collected the stomach contents from 105 tiger sharks — by dissecting dead sharks and washing out the stomachs of live ones — and sent them to Feldheim for analysis.
Some of the samples “smelled pretty bad,” Feldheim says, “because they’re just what the shark was digesting at the time.” Barely digested feathers weren’t too bad, and were easy to extract DNA from. “But others were, you could just tell. You could just take it out of the jar and it just smelled horrible. And those were usually very difficult to extract from,” he says.
Forty-one, or 39 percent, of the sharks contained bird remains from 11 species: eight songbirds (including barn swallows and house wrens) along with white-winged doves, yellow-bellied sapsuckers and the American coot, the only water bird of the bunch, the researchers report online May 21 in Ecology.
“When he started sending me feathers, I just assumed they would all be seabirds like gulls or pelicans or something like that,” Feldheim says.
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