Science

Sooty Tern Vomit Tells a Worrisome Story

Populations of the seabird on Ascension Island have plummeted since the 1950s. A disrupted food chain could be to blame, scientists say.

On a speck of land in the middle of the Atlantic Ocean, Jim Reynolds wades through hundreds of nesting seabirds, ready to collect some vomit.

The Sooty Terns sit tight on their eggs, eyeing the invader weaving through their Ascension Island colony as he reaches down and scoops up an incubating tern to wrap a tracking band around one leg. In protest, the black-and-white bird regurgitates its stomach contents all over Reynolds, who calmly siphons the chunks of partially digested food from his clothes and into a plastic tube.  

In these tubes, Reynolds, an ornithologist at the University of Birmingham in the United Kingdom, collects whole fish, squid, violet snails, and tiny crabs. But he also finds something unexpected for birds that spend their lives at sea: whole green and brown locusts.

Reynolds had studied terns on Ascension for three years without giving their vomit much thought. But while doing fieldwork in December 2012, the scientist noticed that the birds weren't regurgitating the types of food he would expect from healthy Sooty Terns. "The next day I gave everybody sets of specimen tubes," he says. He told the team that "whatever they cough up, if it's fresh enough, collect it."

So, the group began bottling tern vomit—snails, squids, locusts, and all. 

“To find them feeding on locusts is incredible; I’ve never seen that before,” says Chris Feare, a marine ecologist who has studied Sooty Terns in the Seychelles for 40 years and was not involved in Reynold’s research. According to Feare and Reynolds, the unusual menu item could be a sign the birds can’t find enough of their typical prey and are resorting to gobbling up insects instead.

While Reynolds chalked the locusts up to opportunism—locust swarms occasionally visit the island and could make for an easy meal—he was troubled by what he found overall in the diets of Ascension’s terns. The birds seemed to be eating very few bony fish, the meal of choice for healthy seabirds. Perhaps, he thought, studying the birds' dietary history could help him understand why Ascension's Sooty Terns are diminishing.

Since the 1950s, Sooty Tern populations on the island have dropped 84 percent, from 3 million birds to around 350,000. With an estimated global population of approximately 21 million individuals, Sooty Terns as a whole are not in trouble. But the struggles of this isolated population could point to troubling changes deep beneath the surface of the ocean, spurred by commercial fishing in the tropical Atlantic. Reynolds published his results in February in the journal Global Change Biology.  

“For there to be such a dramatic drop of population . . . it’s a staggering event for what is the most numerous tropical seabird in the world,” Feare says.

Sooty Tern. Photo: N. Weber

Oftentimes, introduced predators such as feral cats, which we landed on Ascension in the 1880s, are behind seabird population declines. Indeed, the cats decimated Ascension Frigatebird and Masked Booby populations on the island, and after the cats were eradicated in 2004, both species bounced back. Monitoring data from the Army Ornithological Society suggested that Sooty Terns saw modest population gains post-cat eradication, the population did not fully recover—which means cats alone cannot explain the Sooty Terns’ woes.

In the search for another explanation, Reynolds and John Hughes, a retired army veteran who was Reynolds's graduate student in 2012, began by collecting what the birds so generously provided during field work on Ascension. They also collected feathers (with permission) from museum specimens from the 1890s, 1920s, and 1940s, as well as feathers collected from live birds on the island over the last 20 years.

To deduce diet from feathers, Reynolds’ colleagues Vitor Paiva and Jaime Ramos, from the University of Coimbra in Portugal, used a process called stable isotope analysis to examine each feather’s atomic makeup. This allowed the researchers to trace where the birds forage for prey—in a marine, freshwater, or terrestrial environment—and where that prey is situated on the food chain. The scientists then compared the stable isotopes in the particular food items coughed up by living birds with those in each feather.

Sooty Tern feeding chick with squid. Photo: D. Osborn

This process isn’t perfect, says Richard Inger, an ecologist at the University of Exeter in England, in an email to Audubon. (Inger was not involved in the new study.) Still, the analysis gave the scientists a window into broad changes in the terns’ diets over time—and found that those changes corresponded with the birds’ population decline. Starting in the late 20th century, the terns started eating more squid and small crustaceans, and fewer bony fish. Fish are loaded with protein and fats and as such are a nourishing food source for seabirds and their fast-growing chicks; squid and crustaceans are not a good alternative, says Feare.

Unsated by their discovery, Reynolds and Hughes still wanted to know why the birds had switched over to a diet that left them hungry. The answer, they realized, could be related to declining populations of tuna.

Sooty Terns often rely on marine predators, like tuna, to find fish. The birds spend much of their lives airborne and survive by plucking prey off the ocean surface. (Although the birds live exclusively in marine habitats, Sooty Terns have minimal waterproofing on their feathers and can’t dive below the surface in search of a meal.) Conveniently, fishy predators like skipjack or yellowfin tuna hunt by herding and trapping schools of forage fish up against the ocean surface. As the tuna feast, seabirds snag leaping and wriggling fish from the air. This practice is especially prevalent in tropical waters where seabirds tend to have poor diving abilities, says Sara Maxwell, a marine ecologist at the University of Washington, Bothell. Maxwell was not involved in the new study.  

But human appetite for tuna (and its relatives) is straining populations of these fish worldwide. Reynolds analyzed data from the International Commission for the Conservation of Atlantic Tunas and found that in the eastern tropical region of the Atlantic, catches of yellowfin and skipjack tuna—species Sooty Terns rely on—increased 15-fold from the 1940s to the 1970s and reached a peak in the 2000s. This is part of a global trend: The number of tuna caught each year has risen steadily since the 1950s, with 10.4 million tons caught worldwide in 2008. From 1954 to 2006, tropical tuna populations declined about 60 percent. (Tuna in temperate climates fared even worse in that time, with an average decline of 80 percent.)  

Sooty Tern chicks. Photo: S. Copsey

Climate change could also be playing a role, as warming seas shuffle ecosystems and force tuna to seek cooler waters or find prey elsewhere. If the surface waters around Ascension warmed too much, tuna might move out of the area or stick to deeper, cooler waters, says Maxwell. This would make it harder for the seabirds to find their fishy hunting buddies—if they can at all. The correlation between changes in tuna populations and distribution and the terns’ diets could help explain the massive population drop: The birds are having more trouble catching nutritious fish.

“Why are seabirds so clobbered by human factors?” asks Reynolds, lamenting the cumulative effects of overfishing, climate change, plastic pollution, and oil spills. “I think it’s kind of . . . a perfect storm.”

This year, the British government is supposed to vote on a proposed 440,000 square-kilometer marine protected area to envelope Ascension Island, with fishing restricted in half of that area. Reynolds thinks this kind of protection could be just what the tuna and the Sooty Terns need— although he wouldn’t be surprised if the decision gets put off as the British government deals with some looming political questions involving Brexit.

In the meantime, Jim Reynolds and his team will continue to do their thing on Ascension, returning almost every year to see what they can learn from the birds—and, of course, their vomit. 

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