The global wildlife trade—catching animals in the wild and selling them as pets or food—is a multibillion dollar business that ecologists say is a leading threat to species around the world. But determining the size of the threat is difficult—it's usually a black-market business, and tracking animals costs money and requires locally placed experts.
Enter a team of researchers from Princeton University and Indonesia—and some of the most basic principles of economics. The group identified several species of birds under threat in Southeast Asia, based solely on the prices people were paying for the birds in local markets. In a study published in the journal Biological Conservation, the researchers said analyzing the going rate for an animal, along with the number being sold, can help identify which species are most threatened. (Tweet This)
Previous research cited in the study puts the global wildlife trade at about $10 billion a year and estimates that about one-third of the world's bird species are trapped and sold for food, as pets, or for other purposes. David Wilcove, one of the researchers on the team, found the problem is especially intense in Southeast Asia, where a thriving trade for birds sold in open markets may be threatening several species.
Read MoreAlligator: The newest, cold-blooded food trend
An important hub for the commerce is the city of Medan on the Indonesian island of Sumatra. More than 200 species of birds are sold in markets there—two-thirds of all bird species traded in the country, according to the study. Many birds are caught in other countries and sold to domestic buyers, who value the animals as status symbols.
The trade in birds in Indonesia alone is "huge," said J. Berton Harris, the study's lead author.
"We are talking about thousands of birds being bought and sold all the time," he said. "I have been studying birds my whole life, and I had no idea how big the trade actually was. It's mind-boggling. And when I was in Sumatra, it was hard to find any patch of forest that wasn't being trapped for birds."
The team began by interviewing Indonesian ornithologists to get an idea of which bird species were declining in the wild. They then tried to identify which of them might be disappearing due to the wildlife trade, by counting the birds bought and sold in markets and noting the prices people were paying for them.
Read MoreOceans fill withplastic—and it's hard to remove
The team identified 14 species of Indonesian birds that have been falling in number—nine more than are currently identified by international conservation groups, such as the Red List. That may be because the Red List focuses on species facing global, rather than regional declines, the researchers wrote in their study.
They researchers found that higher-priced birds in the markets tended to be declining in numbers in the wild. They found four species that were going up in price and down in volume—meaning demand for the birds was likely outstripping supply, which suggested these bird were the most threatened.
"We discovered that changes in the prices of the various species being sold in the markets, combined with changes in the numbers of individuals for sale in the markets, provided a pretty reliable and inexpensive way to identify the species that were probably being overexploited," Wilcove said in an email to CNBC.
The study says further research is needed, but the team is confident that its model points in the right direction. For example, their method pointed to two species of birds already known to be under threat from the trade in Asia—the yellow-crested cockatoo and the Bali myna.
Read MoreA brain trick that will make you save more money
The study notes that its method can't replace traditional field monitoring—where scientists attempt to count species in the wild. However, it could be a cheap and efficient tool for researchers when more expensive and time-consuming methods aren't available.
Wilcove said the birds most threatened by the wildlife trade are likely the straw-headed bulbul, the yellow-crested cockatoo and Bali myna, in Asia; the red siskin and Spix's macaw in South America; and the yellow-headed parrot in central Mexico.
"The macaw, in fact, has been driven to extinction in the wild by the bird trade—the only individuals left are those in zoos and private aviaries," Wilcove said.