Crows and ravens owe their world-dominating success to both their body shapes and their proportionately large brains, new research suggests.
The two species undertook a rapid expansion across the planet during their evolution, during which time they left other members of their group, the corvidae, including jays, magpies and jackdaws, behind.
As a result, they are now present around the world, from the Arctic to Australia, Indonesia to Africa, with the only large areas they don’t call home being South America and Antarctica.
While they have long been recognised as intelligent birds that use tools, solve complex abstract problems and even speak a volume of words, it is less well appreciated how diverse they are.
New research by academics at Washington University in St Louis has helped uncover the secret behind these amazing birds’ planetary expansion.
The work highlights two key factors: Crows and ravens’ great flying ability, which allows them to gain access to new places more easily than other birds, and how their big bodies and big brains played an important role in helping them adapt and survive in the new climates they occupied.
“When we think about processes of global diversification, it is important to consider not just the ability to reach new places, but also the ability to survive once you get there," said Carlos Botero, assistant professor of biology in arts and sciences at Washington University.
"Our work suggests that crows and the ravens diversified both quickly and widely because they were particularly good at coping with different habitats.”
The authors said the birds’ 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 US, the scientists found that they have longer wing lengths, bigger body sizes and bigger relative brain sizes compared with other corvids.
The study’s first-author Joan Garcia-Porta, a postdoctoral research associate at Washington University and now a fellow at the University of Barcelona, said: “We hypothesise that these three very convenient combinations of traits are what allowed this group of birds to colonise and diversify across the world."
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, the study said.
"They were smarter than other corvids and, therefore, able to figure out how to live in a new environment, increasing their chances of survival," the research team said.
Their larger body sizes also gave them a competitive advantage over smaller species, helping them to flourish in new places.
“We are excited with these new insights on how these birds were able to do things that even close relatives did not,” Dr Botero said.
“It truly seems that their incredible behavioural 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.”
The result of their readiness to adapt means that crows and ravens experienced high rates of trait evolution and speciation as they moved into the many different environments they encountered during their rapid expansion across the planet.
Arriving in new environments exposed them to new selective pressures – their ability to thrive in the cold Arctic after moving from a tropical rainforest, for example, likely required very different strategies and traits.
“These new environments often favour tweaks to an organism’s phenotype that facilitate survival and overall performance," said Dr Botero.
"That process is often known as optimising selection,” he said, adding that this is often of critical importance in creating new species.
For crows and ravens, these tweaks meant acquiring new beak shapes that did not exist in any other corvids, thereby increasing beak shape variation in the corvidae family.
The scientists also found that they increased body size variation as they colonised new environments.
Dr 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.”
The research is published in the journal Nature Communications.
