One of Charles Darwin’s fabled finch species is slowly disappearing, even as conservationists work desperately to save it.

This “slow-motion extinction,” as a newly published paper puts it, concerns the critically endangered mangrove finch (Camarhynchus heliobates). Native to the Galápagos Islands, the species has found itself plagued by invasive species for decades. The finches have completely vanished from one of their homes, Fernandina Island, and now survive in just one small population of 80 to 100 birds on the west coast of Isabel Island.

Conservationists working to preserve this species have done a good job recently managing the invasive rats and parasitic flies that took a toll on mangrove finch populations, but the new research reveals that the crisis has had a lasting after-effect. The birds on Isabel Island now have half of the genetic diversity that they had a century ago.

This news was actually kind of expected. “We weren't particularly surprised by the low level of current genetic diversity, knowing that the last remaining population has been at critically low population size for quite some time, and nearly decimated by the rats and flies,” says the study’s lead author, Lucinda Lawson with the University of Cincinnati.

In fact, that low diversity may actually be the legacy of historically low population sizes for the species. “It seems clear that mangrove finches were probably always a marginalized species living in the tiny and ephemeral mangrove forests ringing the edges of these two islands,” Lawson says. “Altogether, there probably haven't been sufficient population sizes within the history of this species to maintain significant genetic diversity.”

But the researchers also discovered something else in their genetic tests. It turns out the mangrove finches are breeding and hybridizing with another species, the woodpecker finch (C. pallidus). The two species share an overlapping range and look remarkably alike. Other than the males’ different breast colors and different songs, they’re almost impossible to tell apart.

Again, this hybridization was anticipated because previous researchers had captured a few individuals that seemed to have traits of each species. Still, Lawson says they were surprised by the number of hybrids that they found and that the genetics indicated they’ve been breeding for at least two generations.

Another surprise came when they investigated a nest of chicks being reared by a mangrove finch father. You’d think the nest would have mangrove finch chicks, right? Well, they turned out to be unrelated woodpecker finches.

How did this cross-species parenting come to be? “To purely speculate on the situation, it's possible that the female was a woodpecker finch who shared a nest with a male mangrove finch, yet was involved in extra-pair paternity with a woodpecker finch, so the pure woodpecker babies raised by a mangrove ‘father,’” Lawson says. “It's only one small story within the larger dataset, but it's an interesting anecdote when evaluating mating within a ‘dying’ population.”

Lawson says all of this hybridization probably isn’t hurting the chicks, unlike some other species where hybridization can create sterile or unhealthy young (mules, for example). The two species are closely related, she points out, and they have very small differences between them, mostly focused on small variances in beak shape. Other Darwin’s finch species have also hybridized with less success since the mixing of beak sizes left hatchlings less able to feed themselves.

The hybridized mangrove and woodpecker finches may have even given themselves an advantage over the parasitic fly, Philornis downsi, which sucks the blood out of birds during its larval form. Other research has shown that hybrid nests have fewer flies, so perhaps the hybrids gained a specialization that they could use to defend themselves.

Even though the few mangrove finches that remain continue to face threats, Lawson says she has hope. “Other bird species have bounced back from near extinction, and there's no reason to think that these couldn't as well if their immediate threats are kept in check,” she says, noting in particular the efforts of the Charles Darwin Foundation to control P. downsi levels. “It seems to me that if they can get the flies under control—which they are making some progress on—and keep the rats away, then they may be able to maintain this species and possible see the population grow.”

On the other hand, maybe this slow-motion extinction will result in the mangrove finch continuing to hybridize and being “absorbed” by its related species. “In such a way the mangrove finch would ‘live on’ within the woodpecker finches much as Neanderthals and Denisovans live on within Homo sapiens,” Lawson says. “I don't think this potential merging of species should be seen as a particularly negative outcome if it does come about.”

Beyond that, she says it’s “not unlikely” that Galápagos finch species have been born and reabsorbed several times over the history of the landscape.

If the mangrove finch eventually disappears, it’s probably something that will continue to happen in dribs and drabs, in slow motion, which conservationists from around the world watch closely as they vanish. Or maybe they’ll save it through ongoing captive breeding or other efforts. Either way, it’s likely something we’ll be following this in slow motion for several years yet to come.

Previously in Extinction Countdown: