A birder named Stephen Gosser recently took a stroll through the woods in Lawrence County, Pennsylvania, when he heard what he thought was a scarlet tanager.
These colorful songbirds are famously difficult to spot, so Gosser followed the sound of the bird’s cheerful “chick-burr” call to try to get a good look at it.
When the bird finally came into view, Gosser could tell it wasn’t a scarlet tanager. The bird did not have the brilliant red body of a male tanager nor the delicate yellow plumage of a female. 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.
“I was very confused and perplexed,” says Gosser, who saw the bird in July 2020 and has been birding for over a decade. He had never heard a “rose-breasted grosbeak sound anything like a scarlet tanager.”
Eager to find out the identity of the bird, Gosser reached out to Bob Mulvihill, an ornithologist at the National Aviary in Pittsburgh. With Gosser’s help, Aviary staffers located the bird, collected a small sample of its blood from a vein on its wing for genetic testing, and then then released it back into the wild.
The results of that test, published in Ecology and Evolution in July, indicate that the mysterious bird Gosser found was a hybrid of a rose-breasted grosbeak and a scarlet tanager. A hybrid of these two highly diverged species has never been seen before, and its discovery raises questions about how many other hybrids may be out there waiting to be discovered.
Birds of a feather?
Scarlet tanagers and rose-breasted grosbeaks are both forest-dwelling songbirds that visit the eastern United States during their breeding seasons. The two species last shared a common ancestor over 10 million years ago, making them nearly as diverged as domestic cats and tigers.
Given how distantly related they are, it seems strange that these birds were able to hybridize successfully. However, when it comes to hybridization, birds play by a different set of rules than mammals.
“Bird species can diverge for a long period of time and remain interfertile. That's not as common in mammals,” says David Toews, an assistant professor of biology at Pennsylvania State University who has made a career studying hybridization in birds. While hybridization has occurred between wild greylag geese and Canada geese, which are 12 million years diverged, successful breeding between species this far removed remains rare in birds, Toews says.
The rose-breasted grosbeak and a scarlet tanager have many morphological and behavioral differences. Nevertheless, the fact they were able to hybridize suggests that the two species, though long-diverged, remain genetically similar. (Related: Rare half-male, half-female cardinal spotted in Pennsylvania.)
A perfect mashup
Toews and his colleagues sequenced the genome of the tanager-grosbeak hybrid and discovered that the bird had a rose-breasted grosbeak for a mom and a scarlet tanager for a dad. The hybrid, a one-year-old male, appears to have received a healthy dose of genes from both of its parents—a rose-colored breast and white tummy from its mother and a long, slender beak from its father.
The bird looks like a perfect mashup of its parents, says Daniel Baldassarre, an assistant professor at SUNY Oswego who was not involved with the discovery. “If you just showed me a picture of the hybrid with no context, I probably could have guessed what the parental species were.”
Although the hybrid appeared healthy, it is unclear whether or not the bird will be able to reproduce. It is not uncommon for hybrids to be sterile, especially hybrids of species that are not closely related. Only time will tell if this hybrid will be able to pass on its unique set of genes.
Hybrids such as the tanager-grosbeak perfectly illustrate how difficult it is to define what a species is, says Baldassarre. A species is often defined as a group of organisms consisting of individuals capable of exchanging genes or interbreeding. The existence of hybrids seems to contradict this definition, but it’s not so simple.
“Species are definitely real, but people love to argue about how we define them. It’s a very messy concept,” Baldassarre says.
More to be discovered
Despite their perceived rarity, hybrids of highly diverged bird species “are popping up more and more,” says Baldassarre. “Birds hybridize a lot, so the more that we look for this, the more we’re going to find,” he says. While only 16 percent of bird species have been documented hybridizing with other bird species in the wild, it is possible that hybridization is more widespread than previously thought.
Finding every bird hybrid that spontaneously appears is a daunting task, but with the help of birders like Gosser, it may be possible.
“The way a lot of these hybrids are discovered is that someone was out bird watching and saw a weird bird and was like ‘that’s a hybrid’,” says Baldassarre.
“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 as I found out it’s always good to check out every bird you may run into because you never know....I’ll certainly be keeping my eyes open.”