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How Evolution Helped Crows and Ravens Take Over the World

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In the work led by first-author Joan Garcia-Porta, a postdoctoral research associate in biology in Arts & Sciences and now a fellow in the Department of Genetics at the University of Barcelona, the authors show that crows’ and ravens’ incredible ability to rapidly expand and diversify across the planet was driven by a specific combination of traits.

Using specimens housed in museums across Europe and the U.S., the scientists found that they have longer wing lengths, bigger body sizes and bigger relative brain sizes compared with other Corvids.

“We hypothesize that these three very convenient combinations of traits are what allowed this group of birds to colonize and diversify across the world,” Garcia-Porta said.

Longer wings means higher flying capacities that allowed the birds to disperse across the world. Big brains relative to their bodies suggest that ancestral crows and ravens were behaviorally flexible. They were smarter than other Corvids and, therefore, able to figure out how to live in a new environment, increasing their chances of survival. Their bigger body size also gave them a competitive advantage over smaller species, helping them establish in a new place.

“We are excited with these new insights on how these birds were able to do things that even close relatives did not,” Botero said. “It truly seems that their incredible behavioral flexibility may have played a major role in allowing these birds to survive initial periods of maladaptation and hang in there long enough for selection to catch up and produce a range of new species in the process.”

New homes, new looks

Crows and ravens experienced high rates of trait evolution and speciation as they adapted to the many different environments they encountered during their rapid expansion across the planet. In fact, they had the highest rates compared with any other member of the family Corvidae.

Arrival in a new environment exposed them to new selective pressures. Their ability to live in the cold Arctic after moving from a tropical rainforest, for example, likely required very different strategies and traits.

“These new environments often favor tweaks to an organism’s phenotype that facilitate survival and overall performance. That process is often known as optimizing selection,” said Botero, who emphasized its potential importance in creating new species.

For crows and ravens, that meant acquiring new beak shapes that did not exist in any other Corvid, thereby increasing beak shape variation in the Corvidae family.

The scientists also found that they increased body size variation as they colonized new environments.

Garcia-Porta said: “Thanks to these amazing birds, we now understand a bit more the processes by which animals rapidly expand across the planet and how this geographic expansion translates to the production of new species with new morphologies.”

Reference: Garcia-Porta J, Sol D, Pennell M, Sayol F, Kaliontzopoulou A, Botero CA. Niche expansion and adaptive divergence in the global radiation of crows and ravens. Nat Commun. 2022;13(1):2086. doi:10.1038/s41467-022-29707-5

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