Two Colors of Warblers Mix and Match

Bee-buzzzzzz. From my parents’ deck overlooking their hilltop of restored prairie and ravines of brushy woods in the Driftless Area of northeast Iowa, the buzzy call of a bird cut through the much sweeter cacophony of orioles, grosbeaks, and cardinals.

The quality of the song was strikingly similar to that of the golden-winged warbler I’d heard in Northern Wisconsin just a few days prior, but with only one longer buzzzz at the end instead of a series of buzz-buzz-buzz syllables. I walked closer, and when the bee-buzzzzz came again, I turned toward the sound and caught just a glimpse of a very yellow bird among the emerging leaves. He flew before I could focus my camera, and buzzed cheekily from out of sight.

Blue-winged warblers are moving north, and encroaching on the habitat of golden-winged warblers. Photo by Andy Wilson, used under Creative Commons from iNaturalist.com.

Had I spotted this little guy in the Bibon Swamp, I might have exclaimed in dismay instead of delight. Blue-winged warblers are moving north, and have been for several decades. At the same time, golden-winged warblers have disappeared from many places (declining by 68% since 1966), with the southern limit of their range shifting 340 miles to the north. Their northern limit has also shifted—by 500 miles—into places like Minnesota and Manitoba where they had never been seen before. Where the two birds overlap, they often mate and form hybrids like the Brewster’s warbler I spotted last week in the Bibon Swamp. [Location MAP]

Golden-winged Warbler range contraction. Source

Here's a map of both of the warblers' ranges, with the green showing areas of overlap. Source

This Brewster’s warbler is actually a hybrid between a golden-winged warbler and a blue-winged warbler. The mixing of their gene pool may have an impact on their conservation. Photo by Emily Stone. 

Some scientists worry that this hybridization may ultimately lead to the demise of the golden-winged warbler as a species. That may well be true, although it’s impossible to predict the future. The scientists I talked to all have a pretty philosophical view of the situation.

David Toews, a researcher who compared the genetics of the two species, found that these two members of the Vermivora genus share 99.97% of their genes. Even when scientists find a bird that looks completely like one species or the other, the birds’ DNA reveals evidence of past hybridization. “They could only have gotten this way by hybridizing for a very long time,” he told me. “We like to try and put nature into neat boxes,” he added, “but the distinctions between different species are not always neat and tidy.”

Since both warblers tend to use brushy habitats such as alder swamps and regenerating aspen stands, there doesn’t seem to be a way to prevent the blue-winged warblers from encroaching on their northern cousins if it suits them. “The notion that somehow we’re going to stop them from hybridizing is not within the realm of reality,” Toews added. That might be ok. He thinks we can take a nuanced view and appreciate this “cool evolutionary thing happening in our own back yard.”

Don’t get me wrong, these birds are in dire need of our help, but trying to stop hybridization probably isn’t the answer. What we should do, added Amber Roth, a professor at the University of Maine and co-chair of the Golden-winged Warbler Working Group, is make sure that we manage habitat for the entire Vermivora species complex—which includes golden-winged warblers, blue-winged warblers, and all of their hybrids.

Golden-winged warblers face a myriad of threats as their population both declines and shifts north. Sharing genes with blue-winged warblers may be part of both their demise and their future. Photo by Emily Stone.

As with most species, habitat loss is a critical cause of their decline. Beaver meadows used to provide key habitat, before we trapped them for fur. Wildfires used to create a patchwork of shrubby habitats among larger forests, before we started putting fires out as fast as possible. Even settlers clearing forest openings for farming—and then abandoning them again—in the late 1800s and again in the 1930s was good for the birds, until those openings closed in again. (Those clearings may also have been what brought the two warblers into contact and started this most recent round of increased hybridization.)

Although it often feels unsightly, allowing more aspen clear-cuts within a dynamic, forested landscape could be beneficial for both colors of warblers. Just as essential is making sure that housing developments don’t encroach on important habitat, refraining from draining wetlands, and allowing beavers to do what they do best.

Protecting the warblers’ winter habitat in Central and South America is also critical. While there’s an alliance focused on just that, you can help by choosing bird-friendly coffee, which promotes agricultural practices that really do help birds.

Of course, no amount of habitat conservation will be enough if we don’t get climate change under control. Models predict that with a 2°C increase in temperature, much of the winter habitat in Central America will become unsuitable, and golden-winged warblers will be extirpated from Minnesota, Wisconsin, and much of their current range.

Although hybridization with the blue-winged warbler is often listed as one of the many threats to the long-term survival of golden-winged warblers, it may also provide some hope, at least for the Vermivora genus as a whole. Tom Will, now retired from the U.S. Fish and Wildlife Service, thinks that gene sharing among the two species may allow the best adaptations to surface, and provide a buffer against environmental change. He concluded our interview by advising us all to “Enjoy evolution, admire its processes, and keep birds—all birds—on the landscape!”

Want to find out more? The Working Group is working on their website, but in the meantime The Cornell Lab of Ornithology has put together some really great resources here.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books. Or order it from our friends at redberybooks.com to receive free shipping!

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