During Charles Darwin’s journey to the Galápagos in the 1830s, he noted the existence of “a curious group of finches” found only on the islands. The story that those birds inspired the theory of evolution has long been doubted. But the finches still bear Darwin’s name and are prized by biologists as one of the best examples of speciation—the process by which new species arise.

Now, research suggests that at least some of Darwin’s famous finches might soon be driven to extinction by parasitic flies introduced to them by humans in the 1960s. A paper published this week in the Journal of Applied Ecology blames the parasitic nest fly Philornis downsi for the threat to Geospitza fortis, also known as the medium ground finch.

While adult P. downsi flies are not themselves parasitic, they lay their eggs in bird nests. The eggs hatch into maggots, which feed on both brooding adult finches and their babies. The adult birds are unaffected, but it’s another story with the tiny nestlings. P. downsi kills huge numbers of baby birds among Darwin’s finches and other land birds. The flies can be found on every island in the Galápagos.

“This is like a really bad horror flick,” senior author Dale Clayton, an ecologist and parasite specialist at the University of Utah, says with a rueful laugh. “The babies can’t withstand even one night with these parasites.” The susceptibility of the baby finches has something to do with their size—Clayton compares the newly-hatched birds to peanut M&M's—and their immune responses.

The sight of a parasite-infested G. fortis isn’t just alarming because of the tiny bird’s anemic looks and bulging lesions. It is also a harbinger of the species’ coming extinction. Clayton and his team used five years of field data to predict the species’ long-term viability and found that, in two out of three scenarios, medium ground finches could be gone within the next century.

The team first manipulated the parasite load in actual nests, studying only the medium ground finches, which are the most abundant and have the most accessible nests. They created a control group of nests that they sprayed with plain water, while a second set of nests was sprayed with permethrin, an insecticide that is also used to kill mosquitoes, lice and ticks. The use of treated and untreated nests allowed the team to determine the direct effects of the flies on the birds. They then used that data in models of good, bad and neutral years for the birds’ reproduction and ultimate survival.

The team predicts that if the finches were to run into a series of bad reproductive years in which extreme weather cuts off their food supply, they’d go extinct in about 50 years. A model weighted toward neutral years indicates they’d be extinct within about 80 years. Unsurprisingly, a run of good reproductive luck would spell longer survival for the species: about 100 years. Reducing the number of flies could help the birds hold on even longer, or even prevent them from going extinct at all.

While Clayton’s team focused on medium ground finches, given that there are at least 14 species of Darwin's finches in the Galápagos—and that they are so closely related—the problem likely extends to other species.

For instance, a cousin of the ground finch, the mangrove finch, has already become one of the rarest birds on Earth due to the flies and other predators. If any species is lost, it will be a disaster not just for biodiversity, but for researchers who see the finches as an example of real-time evolution due to their rapid adaptations.

Since the Galápagos so far has its entire native population of birds intact, its importance to biologists is unparalleled. But for Clayton, it goes further than that. “If Darwin’s finches go extinct, it will be because people brought this fly to the islands,” he says. “If the fly had gotten to the island more gradually, perhaps, maybe the birds would have had more time to adapt. But at this point, it’s just an arms race.”

Luckily, there are ways that the finches could beat both P. downsi and time. The team estimates that if nest fly infestations were reduced by just 40 percent, the birds won’t go extinct. Scientists are batting around ideas such as flooding the island with sterile male flies or allowing birds to fumigate their own nests with permethrin.

And don’t underestimate evolution, either: Clayton says there’s an outside chance the finches could still develop their own defenses against the flies. In the meantime, can humans stave off the finches’ untimely demise? “Maybe not. But we’re hopeful.”