Songbirds did not change by inches alone. A new University of Michigan study found their evolution came in rare bursts, often lining up with major shifts in Earth’s climate.

The study focused on Passeriformes, the bird group that includes most songbirds. Researchers used artificial intelligence and a large statistical model to analyze skeletal measurements from modern bird specimens. Their results suggest passerines did not evolve at a steady pace, but through fast bursts of change separated by slower periods.

“This is really important for evolutionary theory because there’s a long history, going back 100 years, that predicts the emergence of new groups, called evolutionary radiations, is often associated with an explosive burst of diversification. Evolutionary theory predicts that adaptive radiations may account for a large portion of the diversity of life on Earth,” said Jake Berv, lead author of the study and postdoctoral fellow in the U-M School for Environment and Sustainability.

“This could be because of a new ecological opportunity, or it could be because a group dispersed to a new continent, resulting in dramatic accelerations in their rate of evolution. The idea is that, over time, there’s less opportunity as evolution proceeds, and so it slows down, and that this occurs in pulses across time. That’s what theory predicts, and that seems to be what we see in the data as well.”

The findings were published in Nature Ecology & Evolution.

To reconstruct the evolutionary history of passerines, the team studied more than 2,000 species and assembled more than 170,000 skeletal measurements. They used Skelevision, an AI tool developed by senior author Brian Weeks’ laboratory with David Fouhey’s laboratory at New York University.

The system photographs specimens in front of a grid that provides a consistent measurement scale. Over a seven-year collaboration, Weeks and Fouhey created an AI model that can measure 12 bones across a bird’s skeleton. The researchers used it to scan and measure more than 15,000 museum specimens, most from the U-M Museum of Zoology. Each specimen can be scanned in about 45 seconds.

Berv also created a statistical method called bifrost, which let the team analyze each species’ complete skeleton instead of looking at individual bones separately. Using that method, the researchers estimated how passerine body shapes changed over about 45 million years of evolution.

“The whole organism is an integrated, complex morphology, and each of the individual pieces is interrelated to every other part in the body,” Berv said. “The question from the model’s perspective is, ‘What is the sequence of evolutionary changes that needs to happen to explain the variation we can see today?’”

The analysis found a period of especially rapid body-shape evolution around 35 million years ago. That burst coincided with the Eocene-Oligocene transition, a period of intense global cooling. The statistical results also found a cluster of evolutionary slowdowns around 15 million years ago, coinciding with another major geological event.

“Our findings have definitely shifted my thinking about how the world works,” said Weeks, associate professor of ecosystem science and management at U-M’s School for Environment and Sustainability. “This pattern we found with rare, big increases in the rates of evolution and lots of small decreases in the rate of evolution is really consistent with a pattern where lineages are exploring new ecological space and changing rapidly to take advantage of that opportunity.”

The researchers also tested the pattern by analyzing the global distribution of the birds in their dataset. They found geography helped predict the average rate of morphological evolution. Bird communities at more extreme latitudes, where seasonal temperatures vary more sharply, tended to include species that evolved faster than those living closer to the equator.

“It looks like there’s a connection between latitudinal gradients and rates of morphological evolution that has been underappreciated,” Weeks said. “I hope our findings will inspire a new integration of rates of morphological change into other big areas of interest, things like the very well-known latitudinal gradients in biodiversity.”

The study also highlighted the value of museum collections. Weeks said artificial intelligence is making it possible to extract information from preserved specimens at a scale that was previously hard to imagine.

“It’s especially clear how important it is to invest in museums when you think about the scale of an analysis like this; it’s so far beyond the scope of what can be done using specimens contributed by an individual collector,” he said. “It’s also fun to imagine what early collectors would make of how we’re using the specimens they collected. I imagine it would blow their minds to learn that a computer has analyzed a photograph of these specimens. It’s just another example of how impossible it is to foresee the full future value of a specimen.”

Berv said the findings may also help scientists think about how species could respond to rapid climate change now.

“Right now, we’re in this moment in human history where there’s dramatic global climate change, and we don’t know what’s going to happen over even a 10-year period, let alone over a 10-million-year period,” Berv said. “To have a chance of understanding the long-term impact of human activity on Earth, we have to study the relationship between events in Earth’s history and evolutionary transitions.”

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