Far West Texas is an ecological crossroads, where the Great Plains meet the desert, the Rockies blend into the Sierra Madre of Mexico. It’s part of what makes our region’s “sky islands” so special. In the Chisos, Davis and Guadalupe mountains, agaves and sotols grow alongside maples and madrones, prickly pears among oaks and pines.
These are also places where creatures mix and mingle. And in the Davis Mountains, that appears to have created a singular pairing between two bird species. One has a claim to being our region’s most legendary avian attraction.
Big Bend National Park is known for its dramatic desert-mountain landscapes. But for birders, its fame lies elsewhere. The Colima warbler isn’t flashy — it’s mostly gray, with a yellow patch on its rear. But birders throng the Chisos each spring to add it to their lists. That’s because Big Bend has long been known as the warbler’s only U.S. abode.
But a 1999 discovery complicated the picture. While surveying The Nature Conservancy’s new Davis Mountains Preserve, local ornithologists Kelly Bryan and Mark Lockwood found Colima warblers of a curious character. In their songs and plumage, they resembled another species — the Virginia’s warbler.
Both birds winter in tropical Mexico. The Virginia’s nests mostly in the Southern Rockies, the Colima from Big Bend south through the Sierra Madre. Could the two be overlapping, and breeding, in the Davis Mountains?
Ari Rice is a Texas Tech PhD candidate.
“I'm very passionate about birds,” Rice said, “and I also have a huge interest in hybrid zones.”
Rice is probing the warbler mystery, with cutting-edge DNA analysis and dogged fieldwork. His small team is capturing warblers in “mist nets.” It requires improvisation.
Rice has used plastic bird “decoys” to attract birds. Most effective, he’s found, is playing a recording of a neighboring male’s courtship song, which activates the warbler’s territorial instincts.
“If it works,” Rice said, “within a couple minutes the male just comes barreling in and hits the net, gets kind of tangled, and then we race to get them out.”
Rice places colored bands on the birds, measures their wings, and collects blood. The birds are unharmed, but analyzing DNA from the blood could be revelatory.
It will show if in fact the Davis Mountains warblers are hybrids. Rice strongly suspects that’s the case. In size and appearance, they’re a blend of the two species. And Rice has observed male warblers continuing courtship singing into the summer, suggesting they’ve failed to find mates. At this lonesome frontier, potential mates may be few, prompting the birds to pair up across species.
DNA could also show when the hybridization began. Is it a recent development? Or was it more common in the Ice Age past, when woodlands were more widespread here?
Colima warblers may be a marquee species, but they’re still mysterious. Birders here can help broaden knowledge.
“If you see any color-banded Virginia’s or Colima warblers, please let me know,” Rice said. “We have no idea where these birds go, even in July or August when they're done breeding. It's thought that they might wander to lower elevations when there's more abundant rainfall, but we really have no idea.”
The warblers rely on oak trees — and the insects that feed on new oak leaves — to raise their young. That makes them vulnerable as the landscape changes.
“It's possible they'll hang on for a little while,” Rice said, “at least until the climate just becomes too dry to support those oaks at all. But it's very possible that they'll wink out of existence at some point in the future due to climate change or further fires and droughts.”
For now, these birds — as unique as the mountains in which they live — are getting the scientific attention they deserve.
