“I love getting huge boxes of blood,” says the genetic ornithologist Rachel Vallender as she pulls open a drawer full of small plastic vials in her laboratory at the Canadian Museum of Nature in Ottawa, where she’s a visiting scientist. Each tube, carefully labeled and organized, holds a blood sample from a single warbler. Whether the bird is actually a hybrid is the question Vallender seeks to answer.
Hybrids of golden-winged and blue-winged warblers are increasingly popping up across the Northeast and into Canada. The physical differences between the mixed progeny and their pure counterparts can be subtle. A bird might, for instance, have the distinctive yellow patches on its wings, the golden head, and the jet-black collar of a golden-winged warbler but with the yellowish belly of a blue-winged warbler. So individual scientists and conservation groups, including Audubon North Carolina and Bird Studies Canada, are gathering samples from across eastern North America and sending them to Vallender, who analyzes mitochondrial DNA in the blood to determine the birds’ genetic history. She examines the shipments she receives in free moments—on nights, weekends, and vacation days from her full-time job with Environment Canada, a government agency. The research is revealing how prevalent this intermingling of genes is and helping bring to light some of the potential dangers it poses.
Records of blue-wingeds spreading into golden-winged territory, hybridizing with them, and gradually replacing them extend back to the early twentieth century. Such mixing isn’t unusual in the avian world: nearly 10 percent of all bird species are known to occasionally interbreed. But the genetic work of Vallender, who has been studying warbler hybridization for more than a decade, backs up the observations of birders and scientists who, during the same time period, have reported growing numbers of hybrids while conducting population surveys. She’s found that in many places across the United States and Canada, hybrids now make up as much as 30 percent of golden-winged warbler populations. “This isn’t just some sporadic event anymore,” she says.
This shift, says Vallender, correlates with the onslaught of climate change. Biologists have long known that habitat loss is a major factor driving blue-winged warblers to expand their range. The bird’s preferred scrubland habitat is disappearing as abandoned farmland reverts to forest. Warming temperatures might be adding additional pressures, causing blue-wingeds to move north in search of cooler climes and into habitat already occupied by golden-wingeds.
For reasons unknown, the golden-winged warblers seem to suffer more from the interaction. While blue-winged populations are experiencing declines, golden-winged populations are plummeting, and scientists are wary of the species’ chances for long-term survival. “If [this decline] continues at the rate it has been going, we could see drastic reductions in their populations or, worst-case scenario, extinctions,” says Vallender. “We need to do this research now.”
What’s happening to the two warblers isn’t unique. Polar bears and grizzly bears are mating, as are different species of everything from butterflies to sharks.
In some instances, it’s clear that climate change is playing a role. More than 1,700 animal species across the globe have shifted their ranges northward and upward in elevation, searching for colder temperatures and following as the plants and other animals they rely on shift as well. Ice sheets and other physical barriers that once kept species apart are disappearing. All of these changes are expected to accelerate as we spew ever more greenhouse gases into the atmosphere, driving up the earth’s temperature.