A homeowner in San Antonio shared a picture to a Facebook group dedicated to bird watching in 2023. The picture was grainy because it was taken through a backyard window, but there was something wrong with the bird right away. Its striking blue coloring was typical of a blue jay, but its heavy, black-patterned, nearly mask-like facial markings gave the impression that they belonged to something completely different. When Brian Stokes, a PhD candidate in the Department of Integrative Biology at the University of Texas, saw the picture, he was genuinely intrigued. There was a problem.
When the bird was eventually captured and its genome sequenced, the outcome was startling. The nuclear DNA revealed a nearly 50/50 split between blue and green jays, with a green jay most likely being the mother. To the best of everyone’s knowledge, it was the first known natural hybrid of those two species. And the probable cause of its very existence? Both species had been subtly pushed out of their historical ranges by climate change until they were now living close enough to interact.
In recent decades, green jays have spread up to 200 miles northward from their previous range near the Texas-Mexico border. Blue jays, on the other hand, have moved westward due in part to temperature changes and in part to urbanization that has displaced them. It was truly uncommon to see a blue jay west of Houston a few decades ago. It isn’t now. According to Stokes and his colleagues, the two species first came into contact due to warming-driven range shifts, and the birds seemed to find each other suitable. “We were really caught off guard that, in some way, mating could occur,” Stokes stated. “That was really surprising to us.”
Although this particular bird is remarkable, it is not a unique occurrence. Across several continents and ecosystems, hybrid wildlife has been observed more frequently. In the Canadian Arctic, polar bears and grizzly bears have been breeding, resulting in what people have come to refer to as “pizzly” or “grolar” bears. This is because polar bears are forced onto land where grizzlies roam due to melting sea ice.
During routine marine surveys conducted in Australia between 2007 and 2009, researchers caught dozens of hybrid blacktip sharks—crossbreeds of Australian and common blacktip species—that were discovered far outside the typical tropical range of either parent species. Already under extreme stress, spotted owls in the American Pacific Northwest are increasingly mating with barred owls that have moved westward, probably along forest corridors made possible by wildfires and rising temperatures.
The pattern is not accidental. In reaction to rising temperatures, more than 1,700 animal species have moved their ranges northward or to higher altitudes. Ice sheets, treeless plains, and cold ocean currents are examples of physical barriers that used to keep species apart but are now changing or vanishing. Species that spent tens of thousands of years evolving in distinct ecological niches are now unexpectedly becoming neighbors and, in certain situations, mates.
Beyond the evident ecological disruption, this is complicated because the results are not consistent. Certain hybrids seem to be quite feasible and might even have advantages in terms of adaptation. Certain hybrid bird species in the Sino-Himalayan mountains demonstrated more resilience to anticipated habitat loss than their non-hybridized counterparts, according to research led by scientists at the Chinese Academy of Sciences and published in Nature Climate Change.
Birds that have been diverging for more than 200,000 years, each acquiring characteristics appropriate for unique mountain habitats, may be able to exchange advantageous genes through hybridization, increasing their chances of surviving future climatic changes as a group. In certain instances, hybridization may be more of a biological pressure valve than an indication of ecological collapse.
However, the dangers are also real. Golden-winged warblers have been drastically declining in the northeastern United States and Canada as blue-winged warblers encroach on their territory, interbreed, and eventually drive them out. According to geneticist Rachel Vallender of the Canadian Museum of Nature, hybrids now make up as much as 30% of golden-winged warbler populations in some survey areas. The golden-winged might not make it through the encounter. A similar story can be found in the barred owl situation in the Pacific Northwest, where a threatened resident population is overrun by more numerous species that arrive from outside due to competition and sheer proximity.
All of this is underpinned by a deeper tension. About 10% of bird species are known to interbreed on occasion, and nature has always rearranged genes during times of climate change, so hybridization is nothing new. However, the changes that took centuries in the past are now occurring in decades, sometimes even years. According to one scientist, contemporary climate change functions more like an abrupt geological event than like gradual pressure. Evolution isn’t always prepared for rapid change, but it can adjust to gradual change.
Naturally, the bird sitting in that backyard in San Antonio was unaware of all of this. It was merely a bird acting in its typical manner. However, it contained a tiny biological record of something more expansive and ongoing: two worlds that drifted into one another, giving rise to organisms that didn’t neatly fit into either.
