This Rare Warbler Is Three Species in One

A warbler discovered in Pennsylvania is the offspring of a hybrid female and a male from a completely different genus

Burket's warbler Cornell Lab of Ornithology

Last May, birder Lowell Burket scored a major find. The Pennsylvania native photographed what he believed was a super-rare, three-species hybrid on his property, a guess that was just confirmed in a new study in the journal Biology Letters.

Hybrids in the animal kingdom happen quite a bit, with closely-related species often producing intermediaries, though in many cases those hybrids are sterile, like mules. In birds, hybrids occur regularly—up to 10 percent of bird species have been caught swinging between species and producing unusual hybrid chicks. Two closely-related New World species in particular, the blue-winged and golden-winged warblers, routinely produce hybrids known at Brewster’s warbler or Lawrence’s warbler depending on their coloration.

That’s what birdwatcher Lowell Burket thought he spotted at a watering hole near his property in Roaring Spring, Pennsylvania, last May. One of the warbler's hopping around appeared to be a Brewster’s, which he'd photographed there before, except when he looked closely at his photos, the markings weren’t exactly right. One thing especially seemed off: The bird had two patches on its breast similar to the color found on a third locally-common species, the chestnut-sided warbler. Since warbler species each have a distinct song, he followed the bird until he heard it sing. The melody the bird belted turned out to be the chestnut-sided warbler’s unmistakable, “Pleased-to, pleased-to meetcha!

Burket sent his images, videos and three-species theory to the Cornell Lab of Ornithology. “I tried to make the email sound somewhat intellectual so they wouldn't think I was a crackpot,” Burket says in a statement. “Having the photos and video helped. Within a week researcher David Toews came down. We found the bird again and collected a blood sample and measurements. It was a very interesting and exciting morning for us. A few days later I got a text message from Dave saying, ‘You were right!!!’”

Ryan F. Mandelbaum at Gizmodo reports the bird’s mitochondrial DNA showed that the its mother was from the Vermivora genus, or golden-sided warbler, and its father was a chestnut-sided warbler, part of the genus Setophaga. When Toews posted his preliminary finding online, however, others encouraged him to take the research further.

“We looked at the genes that code for different warbler colors,” Toews says in a statement. “This way we could recreate what the hybrid’s mother would have looked like – the avian equivalent of a detective’s facial composite, but generated from genes. We confirmed that the mother would have looked like a Brewster’s warbler and the father was a chestnut-sided warbler.”

That confirmed that the new bird‚dubbed Burket’s warbler after the birder who made the initial find—was an extremely rare three-species hybrid.

So, why would a bird of one species mate with another in an entirely different genus? “It could have just been a mistake (these things happen),” Toews tells Forbes via email, “although it could be that the number of suitable mates is so low (Vermivora warblers are declining throughout the Appalachians) [that] she was making the ‘best of a bad situation’.”

In fact, hybridization between blue-winged and golden-winged warblers is on the rise. The number of golden-winged warblers has plummeted by 66 percent since the 1960s and by 98 percent in the Appalachian region. The main culprit is habitat loss in both its breeding ground in North America and on its wintering grounds in Central and South America. But hybridization is also leading to population loss. Instead of foregoing breeding when they can't find a mate of the same species, the golden-winged warblers are instead pairing up with blue-winged warblers.

“Choosing to mate with a male that isn’t perfect might be better than no mate at all!” Toews tells Forbes.

The hybrid also tells researchers something about warblers in general. While many species eventually evolve away from each other to a point where they can no longer reproduce viable offspring, evolution in the warblers seems to be different.

“It tells us that warblers in general appear to be reproductively compatible over millions of years of independent evolution,” Toews tells Mandelbaum at Gizmodo.“The things that really define them, their distinct colors and their songs, are likely mating barriers, and that they don’t interbreed because they can’t, but because they choose not to.”

The big question now is where the three-species hybrid will look for love once its ready to mate. Burket will continue to keep an eye on the nearby watering hole to find out.

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