Recently, while walking through the woods in Lawrence County, Pennsylvania, birder Stephen Gosser heard a bird he assumed to be a scarlet tanager.
However, Gosser heard a bright “chick-burr” call and decided to follow it in the hopes of catching a glimpse of the elusive colorful songbird.
Gosser knew the bird wasn’t a scarlet tanager as soon as it fully entered his field of vision. Neither the bright red of a male nor the soft yellow of a female tanager could be seen on this bird. This bird had brown wings, a speckled chest, and a patch of red feathers on its throat not unlike that of a rose-breasted grosbeak.
Gosser, who has been birdwatching for more than a decade, saw the bird in July 2020 and was “very confused and perplexed” by it. When asked if he had ever heard a “rose-breasted grosbeak sound like a scarlet tanager,” he said no.
Gosser contacted Bob Mullvhill, an ornithologist at Pittsburgh’s National Aviary, to learn the bird’s name. Mullvhill, with Gosser’s assistance, tracked down the bird, took a blood sample from a vein on its wing for genetic testing, and then released the animal back into the wild.
The results of that analysis, which were published in July in Ecology and Evolution, suggest that the mysterious bird Gosser discovered was a cross between a rose-breasted grosbeak and a scarlet tanager. There has never been evidence of a hybrid between these two extremely different species before, so the discovery of one begs the question of how many other hybrids there might be out there.
People with similar traits?
Songbirds like the scarlet tanager and the rose-breasted grosbeak are found in the eastern United States during the breeding seasons because of the abundance of suitable forest habitat. Over 10 million years have passed since the two species shared a common ancestor, putting them on par with the differences between housecats and tigers in terms of evolutionary distance.
The fact that these bird species were able to successfully hybridize despite their distant genetic ties is surprising. However, birds follow a different set of rules when it comes to hybridization.
Images of endangered species, including the monarch butterfly (Love Exploring)
Long-term divergence without loss of fertility is possible in bird populations. David Toews, an assistant professor of biology at Penn State who has spent his career studying hybridization in birds, says that this is much less common in mammals. Successful breeding between species so far removed is uncommon in birds, according to Toews, but it has happened between wild greylag geese and Canada geese despite a 12-million-year divergence.
The genome of the tanager-grosbeak hybrid was sequenced by Toews and colleagues, who determined that the bird was the offspring of a rose-breasted grosbeak and a scarlet tanager. The hybrid, a male of one year old, appears to have inherited characteristics from both parents, including its mother’s pink breast and white belly and its father’s long, slender beak.
Assistant professor at SUNY Oswego and outside observer Daniel Baldassarre says the bird is a beautiful hybrid of its parents. “I think I could have guessed what the parental species were if you just showed me a picture of the hybrid without any background information.”
Although the hybrid bird appeared to be in good health, its reproductive potential is still unknown. Sterility in hybrids is not uncommon, particularly in those involving species with a distant evolutionary relationship. Whether or not this hybrid is successful at reproducing its distinctive traits remains to be seen.
Baldassarre argues that it is complicated to determine what constitutes a species due to examples like the tanager-grosbeak. It is commonly agreed that a species is any collection of organisms that can interbreed and/or exchange genetic material. This definition appears to be in conflict with the existence of hybrids, but the issue is more nuanced than that.
Species exist unquestionably, but definitions of them tend to spark heated debate. Baldassarre describes the idea as “very messy.
There is more to learn.
Baldassarre claims that hybrids between previously undiscovered, widely separated bird species are “popping up more and more” despite popular belief to the contrary. The more we look for it, the more we’ll find it, he says, because “birds hybridize a lot.” It is possible that hybridization is more commonplace than previously thought, despite the fact that only 16% of bird species have been documented hybridizing with other bird species in the wild.
It’s a tall order for birders like Gosser to track down every naturally occurring bird hybrid, but it just might be doable.
Baldassarre claims that “the way a lot of these hybrids are discovered is that someone out bird watching saw a weird bird and was like ‘that’s a hybrid.
“To find a hybrid bird, especially one where the two parent species were never known to mate is most likely a once-in-a-lifetime find,” says Gosser. But I learned that you should examine any bird you encounter, because you never know what you might find.