When a baby shark in the Gulf of Mexico threw up, an unlikely team of scientists came together to see what they could learn from the nasty mess.
What they found sounds impossible: baby tiger sharks eat songbirds that live on land.
Marcus Drymon, an assistant professor with the Mississippi State University Extension Service and a marine fisheries specialist with Mississippi-Alabama Sea Grant Consortium, described it as “a unique relationship between a marine predator and a terrestrial consumer.”
The young shark that started the mystery in 2009 was not sick. Drymon had caught, tagged and released the shark as part of a routine survey to determine the population size and species of marine life in the northern Gulf of Mexico.
But this particular shark lost its lunch when brought on board for the brief scientific procedure. When “lunch” was a pile of feathers, in true scientific fashion, Drymon bagged it up, stuck it in the freezer and pulled it out later for examination.
It turned out that feathers no longer attached to a bird were much harder to identify than Drymon imagined, so he turned to a colleague in Chicago who was able to extract their DNA.
The geneticist made an interesting discovery.
“None of the birds were seagulls or pelicans or any type of marine bird. They were all land birds,” Drymon said.
Sharks are known to barf in high-stress situations, and they sometimes hurl simply to clear their stomachs of the detritus of their voracious appetites. After the first incident with the shark tossing its cookies, that is, feathers, Drymon decided to see what other sharks had been eating.
He devised a method to deprive captured sharks of their most recent meal with no long-term ill effects. He gathered data on the stomach contents of numerous sharks over the next few years.
“When we finally started to look at the data, we had 100 tiger sharks, and basically four in 10 had bird remains in their stomachs,” Drymon said.
These feathered shark snacks included barn swallow, eastern kingbird, house wren, common yellowthroat, marsh wren, eastern meadowlark, swamp sparrow, brown thrasher, white-winged dove and the yellow-bellied sapsucker. There was one American coot, a water-dwelling bird, among the mix.
Ever the researcher, this discovery left Drymon with a list of questions: How does a shark catch a bird? Can you identify a bird from just partially digested feathers? How does a land-dwelling bird end life as a shark meal in the Gulf of Mexico?
The shark vomit even became a lunch-time topic in Biloxi at the MSU Coastal Research and Extension Center, and answering these questions became a cross-disciplinary effort between a fisheries ecologist, a geneticist and a bird ecologist. As Drymon put it, “a shark guy, a genetics guy and a bird girl” put together the puzzle they could not have completed alone.
“The majority of these sharks were very young tiger sharks, and the story ended up being about the propensity of baby tiger sharks to eat land dwelling birds,” Drymon said. “It was very surprising and very puzzling. Forty percent of these sharks are eating birds that don’t live over the water.”
“The majority of these sharks were very young tiger sharks, and the story ended up being about the propensity of baby tiger sharks to eat land dwelling birds. It was very surprising and very puzzling. Forty percent of these sharks are eating birds that don’t live over the water.” ~ Marcus Drymon
Auriel Fournier, now director of the Forbes Biological Station in Havana, Illinois, was then a post-doctoral researcher working in Biloxi with Extension. She studies bird migration and soon became involved in these conversations.
“What I wanted to figure out was if these were just kind of one-off events, things that didn’t happen with any kind of pattern, or if the sharks were eating the birds during the peak of migration, when the most birds were passing through the area,” Fournier said.
“If we found that it was around the time of those peaks, it would then imply that there was more to this story than just a few one-off events,” she said.
Knowing she needed specific bird migration data, Fournier turned to eBird.org, a community science database where anyone who observes birds can submit their observations. By accessing this data compiled from millions of observations from around the world, she was able to determine when migration for each bird species peaked in the area.
“Crowd-sourced community science data is perfect for these kinds of questions where we want to understand what is happening with bird migration at large, spatial scales, like counties or the coastline of Mississippi and Alabama,” she said.
There was still the question of how the birds ended up in the water when their goal was to fly across the entire Gulf of Mexico in 12 or more hours to reach the Yucatan Peninsula and continue to points farther south.
“Migration is one of the most hazardous things that birds do, and I’m not surprised that so many don’t make it,” Fournier said. “Most of these birds are smaller than a softball. Birds migrate twice a year because the advantages of plentiful food supply at their breeding grounds are enough to justify the costs.”
Fournier put her bird migration data together with Drymon’s shark-vomit data, and they found an interesting pattern.
“We saw peaks in the relative abundance of these birds around the time we would see those birds in the tiger shark stomachs,” Drymon said. “We think inclement weather patterns would come up, disrupt the birds’ migration patterns and they would get disoriented and fall into the water.”
Since billions of birds migrate across the Gulf of Mexico twice a year, if even a fraction of a percentage of them are lost at sea, that still provides a significant food source for foraging baby sharks.
What started out as a quest to satisfy simple curiosity has some interesting implications.
“We better understand the foraging inclinations of tiger sharks,” Drymon said. “Sharks in general are slow-growing and late to mature, which makes them over-susceptible to harvest. Knowing where they are at certain times of the year is important to protecting them.”
Another finding is that the northern Gulf of Mexico—the area off the coast of Mississippi and nearby states—is a significant pupping ground, or nursery area, for baby sharks.
Fournier is curious about the proportion of birds that fail to complete their migration each year, and whether sharks seek out birds in the water as a preferred food source or are simply opportunistic in their dining.
“Those questions would be fun to try and answer with a future project,” Fournier said.
Biodiversity in the Gulf region supports ecosystem, economies
Not only is the Gulf of Mexico a significant pupping ground for tiger sharks, the area has tremendous biodiversity in both the water and air.
In the skies, the states bordering the northern Gulf of Mexico are very important habitat for birds ranging from the tiny hummingbird to large raptors.
Gulls are among the most frequent marine bird species living on the Gulf, but Auriel Fournier, a former post-doctoral researcher with MSU Extension, said this group is difficult to observe and study as many rarely come near shore.
“Marine birds are a group of birds we know the least about in the Gulf of Mexico,” she said.
Of the 400 or so land species of birds that migrate across the Gulf, the red-winged blackbird is probably the most plentiful. The smallest is the ruby-throated hummingbird which, at 0.11 ounces, makes the trek across the Gulf twice a year from its native range across North America to Central America.
“The Gulf has really high avian biodiversity compared to other large water bodies, and most of the migratory birds in North America east of the Rocky Mountains either pass over or around the Gulf of Mexico during migration,” Fournier said. “That means the habitats on the coast and the open water habitat are really important for those birds twice a year.”
The Gulf of Mexico has substantially higher biodiversity than any other marine region of the United States, including 1,541 different fish species. Marcus Drymon, an assistant Extension professor at MSU’s Coastal Research and Extension Center, said this includes fish classified in 45 orders, 237 families and 736 genera.
There are about 45-50 shark species known to live in the Gulf, and another 40 or so skates and rays. Dolphins and manatees also call the Gulf home.
“The incredible biodiversity in the Gulf of Mexico supports healthy ecosystems and thriving economies,” he said.
By Bonnie Coblentz