Birdsong reveals a rare hybrid coupling.
Stephen Gosser, a self-described “diehard birder,” was out in the Western Pennsylvania woods in June of 2020 when he believed he heard the singing of the elusive and breathtakingly beautiful scarlet tanager. The blood-red bird, which has black wings and a tail, is a favorite among birders because of both its beauty and rarity since the species prefers to remain hidden high in the forest canopy.
When Gosser finally located the songbird, he found what looked like a rose-breasted grosbeak but sounded just like a scarlet tanager. He snapped a few photos and called for backup; shortly after, a team from National Aviary in Pittsburgh arrived to catch the bird and collect a blood sample.
In order to follow up on Gosser’s tip, a group of scientists headed by Penn State was able to identify the specimen as a unique hybrid bird, whose relatives haven’t congregated in the same breeding location or lineage for 10 million years. Their findings were recently published in the journal Ecology and Evolution.
“I love this story because it starts with a little mystery and ends with a surprising discovery,” said David Toews, lead author of the study and assistant professor of biology at Penn State.
The story begins with an unlikely encounter between a female rose-breasted grosbeak and a male scarlet tanager. Since the two species favor different habitats, experts are still unsure of how and where they met. Tanagers normally favor the canopy cover of mature forests, whereas rose-breasted grosbeaks prefer the open spaces along woodland edges. According to Toews, the two species have been on different evolutionary trajectories for at least 10 million years—until now—because of their vastly different nesting preferences.
The researchers determined that the bird Gosser spotted was the healthy, 1-year-old male offspring of a rose-breasted grosbeak and scarlet tanager, the first-ever documented hybrid of its kind. Yet, his origin story was largely a mystery.
Luckily, Toews had a host of techniques available for solving just this type of mystery. From the blood sample, they could obtain a small sample of DNA. The combination of audio and genetic material would get them as close as they could to solving the mystery of the bird’s genesis.
Their methodology relied on analyzing both nature and nurture. For the most part, songbirds learn to sing from their fathers. Their vocalizations can reveal how and by whom they were raised.
“We knew Mom was there, she was the one who laid the egg and sat on the nest,” Toews said. “It’s still not obvious to us where that would have been, because the two species prefer such different habitats. Wherever it was, her pair either stayed around long enough for the young offspring to learn his father’s song or learned a neighborhood scarlet tanager song.”
The researchers used a method called bioacoustic analysis to confirm the vocalizations they captured did, in fact, match the song of a scarlet tanager — revealing that the hybrid likely learned to sing from his father.
“Something people may not understand is that when we analyze birdsongs, we’re not actually listening to them. We’re looking at them,” said Toews. “We’re looking at wavelengths of the sound — or the ‘spectrogram’ is a more accurate term — and we’re actually measuring visual components of a soundwave to analyze the song.”
With the vocalizations confirmed, the team turned to genomic sequencing to track the genetic ancestry of the hybrid. Nature confirmed what nurture had already revealed, a grosbeak mother and tanager father.
“We used the same tools that we’ve used to identify other hybrids, but we typically have more ambiguous answers that are a bit more esoteric,” said Toews. “In this case, we identified the species. We know who the parents were and we have a somewhat satisfying conclusion at the end. I find this story resonates with more than just your average ornithological nerd like myself.”
Reference: “Genetic confirmation of a hybrid between two highly divergent cardinalid species: A rose-breasted grosbeak (Pheucticus ludovicianus) and a scarlet tanager (Piranga olivacea)” by David P. L. Toews, Tessa A. Rhinehart, Robert Mulvihill, Spencer Galen, Stephen M. Gosser, Tom Johnson, Jessie L. Williamson, Andrew W. Wood and Steven C. Latta, Ecology and Evolution.
The study was funded by startup funds from Penn State’s Eberly College of Science and the Huck Institutes of the Life Sciences.
Bird handling was approved by the Institutional Animal Care and Use Committee of the National Aviary and Pittsburgh Zoo and PPG Aquarium.