In the sunbaked Galápagos Islands, a male finch perches on a branch, hearing what sounds like another bird’s song. But something’s not quite right. The song is slower and simpler, like a familiar tune played at the wrong speed. The bird cocks its head, ruffles its feathers, but stays put. It’s just been fooled by a scientific experiment that’s helping reveal how ecological changes might drive the evolution of new species.
A new study published in Science offers insights into how Darwin’s famous finches evolved into different species. The study used an innovative experiment to understand a fundamental question: how do new species form? While the answer is never simple, this research reveals one way it might happen: through small changes in beaks that lead to changes in song, eventually causing birds to no longer recognize each other as the same species.
The research began with fieldwork on the Galápagos Islands in 1999, where lead author Jeff Podos, a behavioral ecologist at the University of Massachusetts Amherst, has studied these famous birds, specifically Darwin’s medium ground finch (Geospiza fortis), over many field seasons.
Song of the Darwin’s medium ground finch (Geospiza fortis).
Despite their historic significance, Darwin’s finches are rather clumsy. “They’re actually terrible flyers, really poor flyers,” Podos tells Mongabay.
Still, handling these living pieces of evolutionary history never lost its magic, he says. “The very first time I caught a Darwin’s finch, I said ‘You know who you are. You’re famous,’” Podos recalls. “‘You may just think you’re a little bird, but you’re a Darwin’s finch.’ And that excitement has never gone away.”
The study builds on three basic ideas: droughts force birds to develop bigger beaks to crack tough seeds; birds with bigger beaks can’t sing as quickly or with as much variety; and birds use songs to choose their mates.
These principles were demonstrated during a landmark 1977-78 drought study on Daphne Major Island in the Galápagos, when researchers observed that finches with larger beaks were more likely to survive during severe drought conditions because they could crack the remaining tough seeds.
“From one year to the next, most of the birds died during the drought. The ones that survived had larger beaks, and when they bred, the babies then had larger beaks,” Podos says.
The changes in beak size were remarkably precise. Each drought event increased beak depth by about 0.49 millimeters. These small changes in beak size have unexpected consequences for how the birds communicate.
Larger beaks make it harder for birds to produce rapid, complex songs, much like how it would be difficult to “play the ‘Flight of the Bumblebee’ on a tuba,” Podos said.
“If you have a really huge beak, it’s harder to open and close it really rapidly, and it’s also harder to open it more widely. So that means your song is going to sound different,” Katie Schroeder, a co-author of the study who worked on the research as a Ph.D. student at the University of Massachusetts Amherst, tells Mongabay.
To understand how these changes might affect species formation, the team created simulated “ghost of finches future” songs using sound analysis software. They took recordings of real finch songs and modified them in two ways: inserting tiny spaces between notes to slow down the song’s pace, and using frequency filters to narrow the range of pitches, creating the kinds of changes that would result from larger beak sizes.
They created versions simulating what songs would sound like after one, three, or six drought events. The six-drought scenario was chosen because it would produce beak changes similar to the actual differences seen between existing species.
Testing these modified songs required careful experimental design. On the Santa Cruz Island in the Galápagos, researchers carefully captured finches, measured their beaks, then marked them with colored leg bands for future identification before releasing them. The finches, which aren’t migratory, mostly stay in their territories, making them ideal subjects for the playback experiments.
To test their modified songs, researchers set up speakers in the territories of 12 male finches. Each bird heard four different versions of a song: a normal finch song and three modified versions simulating what that song would sound like after one, three, and six droughts. The team played one version per day, with rest days in between to ensure each bird’s response was fresh.
Male finches, it turns out, are fiercely territorial during breeding season. When they hear another male’s song, they typically act within seconds, searching for the perceived intruder. “It’s really fun to do,” Podos says. Some experiments even used taxidermied birds as decoys. When revealed, “They’ll come in looking for the intruder … that territory holder will come in and beat the crap out of your mount,” Podos says.
When hearing normal songs from their own species, males would fly off their perch within about 10 seconds to investigate. The differences were striking and measurable: birds took nearly four times longer to respond to six-drought songs compared to normal ones, and their flight rates decreased by 31%. This suggests they no longer recognized these modified songs as coming from their own species.
With six-drought songs, “they pay attention, “Podos says, “but they don’t leave the perch for about a minute. They’re much more chill about it.”
They also maintained greater distances from speakers playing the most modified songs. While there were some complications in two trials, excluding these would have actually strengthened these findings.
“The study is very well done,” Tim Wright, a biology professor at New Mexico State University who studies bird songs but wasn’t involved in this research, tells Mongabay. “[It] ingeniously projects the effects that successive droughts would have on beak sizes via changes in seed banks, and then simulates the types of songs that could be sung by males with the resulting larger beaks.”
Podos also hoped to test female finches, since it’s the females that ultimately choose mates. However, this proved challenging as females are more cautious. “They skulk around, and they don’t typically approach males,” Podos says. “They’ll only approach a male if they’re really ready.”
While many scientists warn that climate change will drive species to extinction, a very real threat, this study suggests that climate pressures might also create new species. When environmental changes force animals to adapt, these adaptations can cascade into differences in behavior and communication that eventually split one species into two. However, the current pace of climate change may be too rapid for many species to adapt through natural selection, which typically occurs over many generations.
While the study used simulated songs representing six-drought events, both Podos and Wright caution against oversimplification, or concluding that “six droughts makes a new species.” Real climate patterns are complex, with droughts typically alternating with rainy periods. However, Podos says, these patterns may change as the climate crisis continues.
This article by Liz Kimbrough was first published by Mongabay.com on 8 November 2024. Lead Image: A male Darwin’s medium ground finch (Geospiza fortis). Photo courtesy of Andrew Hendry.
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