It all started when a small tiger shark barfed up a bunch of feathers.
Marcus Drymon is a fisheries ecologist at Mississippi State University in Biloxi. He had been catching sharks as part of a long-term monitoring program in the north-central Gulf of Mexico. Typically, he and his colleagues would pull a shark out of the water for only about 90 seconds. That’s enough time to weigh and tag the fish before releasing it. But perhaps stressed by this experience, one tiger shark back in 2010 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,” Drymon says. Then he took it back to the lab.
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 which bird species the feathers had come from. So he reached out to an old friend. This was molecular ecologist Kevin Feldheim at 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. This technique lets scientists identify a species based on a short strand of mitochondrial (My-toh-KON-dree-ul) DNA. The analysis showed that the feathers came from a songbird called the brown thresher.
Intrigued, Drymon searched through scientific journal articles. He found a handful that had reported tiger sharks eating terrestrial birds. And so he decided to investigate how common this bird eating was. He and his colleagues continued their monthly surveys of sharks from 2010 to 2018. Whenever possible, they collected the stomach contents of tiger sharks — by dissecting dead sharks or washing out the stomachs of live ones. In all, they got stomach contents from 105 tiger sharks and sent them to Feldheim to analyze.
Some samples “smelled pretty bad,” Feldheim recalls. Barely digested feathers weren’t too bad. He also found it easy to extract DNA from them. Other samples were harder to work with. “You could just take it out of the jar,” he says, “and it just smelled horrible. And those were usually very difficult to extract from.”
Free meals fall from the sky
Forty-one of the sharks (39 percent) contained bird remains from 11 species. These were eight songbirds (including barn swallows and house wrens) along with white-winged doves, yellow-bellied sapsuckers and the American coot. That last was the only water bird in the bunch. The researchers reported their findings 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.
The part of the Gulf where Drymon works is near the last spot migratory birds can land before they take off for Mexico in the fall. It’s also the first spot where many can land in spring after their marathon flight across the water. Drymon at first thought that the birds might be the ones that don’t quite make it to land in spring. But he looked at the timing of when the birds showed up in the sharks and could see that most of the birds were eaten in the fall.
“That,” he says, “was a puzzle.”
To make sense of it all, Drymon reached out to a Mississippi State colleague, Auriel Fournier. She is an ornithologist who is now director of the Forbes Biological Station in Havana, Ill. They consulted eBird, an online database of bird sightings. They also hashed out their data with other colleagues over lunches.
These birds must take off for their journey south and then get caught when the weather unexpectedly turns nasty, the researchers decided. And unlike ducks and other water birds, the species found in the sharks can’t keep themselves dry once they hit the water. So if they’re knocked out of the sky by something like a storm, they become shark bait, Drymon and his colleagues now propose.
“It made sense within what we know about bird migration,” Fournier says. “It’s just a piece of the puzzle that we didn’t know about.”
So, are these birds feeding other species in the Gulf, too?
No, Drymon says. “I’ve looked at the stomachs of so many other species of shark, so many other species of bony fish. And none of them eat these terrestrial birds — or marine birds, for that matter — the same way that the tiger sharks do.”
The birds’ fall migration does coincide with a peak in the young tiger-shark population in the north-central Gulf of Mexico. And about half the sharks that had birds in their bellies were in fact baby sharks. Drymon and his colleagues now think that mama sharks have figured out that the area is a good time and place to give birth. It gives their young access to an easy-to-obtain food source before they’ve fully honed their hunting skills. Says Drymon, “Why not take advantage of these deceased little songbirds that are apparently littering the surface of the ocean?”
That idea fits with tiger sharks’ will-eat-anything reputation. “I don’t like it necessarily when people say tiger sharks are the trash cans of the sea,” Drymon says. “That carries a kind of negative connotation.” He likens them to like teenage boys, gobbling up whatever food you put in front of them — even terrestrial birds that nothing else will apparently touch.
While Drymon, Feldheim and Fourier discovered something interesting, they all agree that one of the best parts of this study was their collaboration. “I saw something interesting, but it didn’t make sense to me,” Drymon says. He couldn’t figure it out on his own, so he brought in experts from in other fields. This let him do work outside his own field, “which is just a fun thing to do,” he says. Plus, “if you’re lucky, you get to collaborate with friends.”
colleague Someone who works with another; a co-worker or team member.
database An organized collection of related data.
deceased An adjective that describes someone who has died.
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.
fishery A place where fish are raised commercially (for sale) or a site where large numbers of a fish species naturally congregate, making it a place where commercial fleets come to harvest that species.
gull A family of long-winged and relatively thick-bodied shoreline birds. Most are gray and white with webbed feet. They tend to be very vocal.
marine Having to do with the ocean world or environment.
migration (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.
migratory An adjective for species that travel long distances each year, along fairly regular paths, to to find food or more hospitable conditions (such as better weather). Such travels are known as migrations.
mitochondrial DNA DNA passed on to offspring, almost always by their female parent. Housed in mitochondria, this DNA is double-stranded but circular. It’s also very small, only possessing a small share of the genes found in the main package of DNA, the material found in a cell’s nucleus.
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
ornithologist A scientist who studies birds, their behaviors and ecosystems.
population (in biology) A group of individuals from the same species that lives in the same area.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sharks A family of primitive fishes that rely on skeletons formed of cartilage, not bone. Like skates and rays, they belong to a group known as elasmobranchs. Then tend to grow and mature slowly and have few young. Some lay eggs, others give birth to live young.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
survey To view, examine, measure or evaluate something, often land or broad aspects of a landscape. (with people) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.
tag (in conservation science) To attach some rugged band or package of instruments onto an animal. Sometimes the tag is used to give each individual a unique identification number. Once attached to the leg, ear or other part of the body of a critter, it can effectively become the animal’s “name.” In some instances, a tag can collect information from the environment around the animal as well. This helps scientists understand both the environment and the animal’s role within it.
terrestrial Having to do with planet Earth, especially its land. Terra is Latin for Earth.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.