Birdsong unveils a rare hybrid clutch that’s 10 million years in the making

In June 2020, Stephen Gosser, a self-proclaimed “die-hard birder,” was out in the woods of western Pennsylvania when he thought he heard the song of the elusive and strikingly beautiful scarlet tanagers. The crimson bird with black wings and black tail is popular with bird watchers for both its beauty and rarity, as the birds prefer to hide high up in the tree canopy.

When Gosser finally located the songbird, he saw what appeared to be a rose-breasted hawfinch, but it sounded exactly like a scarlet tanager. He snapped a few photos and called for backup — a team from Pittsburgh’s National Aviary arrived shortly after to capture the bird and draw a blood sample.

Taking Gosser’s tip further, a team of researchers led by Penn State was able to use a combination of genome sequencing and song analysis to identify the specimen as a rare hybrid bird whose ancestors didn’t share the same breeding site or lineage for 10 million years. Her work was recently published in the journal ecology and evolution.

“I love this story because it begins with a little mystery and ends with a surprising discovery,” said David Toews, lead author of the study and an assistant professor of biology at Penn State.

The story begins with a very unlikely encounter between a female rose-breasted hawfinch and a male scarlet tanager. How and where they met remains a mystery to researchers, as the two species prefer different habitats. Tanagers usually prefer the treetops of old forests, while Rose-breasted Hawfinches are happy outdoors along the forest edges. Toews explained that the two species have such different nesting preferences that they’ve been on independent evolutionary trajectories for at least 10 million years — until now.

The researchers determined that the bird Gosser discovered was the healthy 1-year-old male offspring of a rose-breasted hawfinch and a scarlet tanager, the very first documented hybrid of this species. However, its origin story has been largely a mystery.

Luckily, Toews had a variety of techniques for solving these types of puzzles. They were able to obtain a small DNA sample from the blood sample. The combination of audio and genetic material would bring them as close as possible to solving the mystery of the origin of the bird.

Their methodology relied on analysis of both nature and nurture. Songbirds mostly 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. “We still don’t know where that was because the two species prefer such different habitats. Wherever they were, their pair either stayed close long enough for the young offspring to learn their father’s song, or learned the song of a neighborhood Scarlet Tanager. “

The researchers used a method called bioacoustic analysis to confirm that the vocalizations they detected did indeed match a scarlet tanager’s song – revealing that the hybrid likely learned to sing from its father.

“What people might not understand is that when we analyze birdsong, we don’t really listen to them. We look at them,” Toews said. “We look at the wavelengths of the sound — or ‘spectrogram’ is a more accurate term — and we actually measure the visual components of a sound wave to analyze the song.”

With the vocalizations confirmed, the team turned to genome sequencing to trace the hybrid’s genetic lineage. Nature confirmed what upbringing had already revealed, a hawfinch mother and a tanager father.

“We used the same tools that we used to identify other hybrids, but we usually have ambiguous answers that are a bit more esoteric,” Toews said. “In this case, we identified the species. We know who the parents were and we have a reasonably satisfactory conclusion at the end. I think this story appeals to more than your average ornithological nerd like me.”