For the ability to soar among the clouds, birds made an evolutionary compromise: When their forelimbs became wings, they no longer had the option of using those limbs to eat, build homes, and care for their young. Many species opted to use their beaks for those tasks instead.
But some birds also evolved to be “pedal dexterous”—able to accomplish tasks with their feet that other animals undertake with nimble hands. Now, researchers have finally discovered where that handy trait got its roots: in a common ancestor of parrots and raptors that lived in trees more than 60 million years ago.
The research, published today in Communications Biology, reveals that nature found “a remarkable end-around,” as described by Stephen Brusatte, a paleontologist at the University of Edinburgh who wasn’t involved in the study. “Evolution molded the feet of birds into multipurpose tools that … almost become pseudohands,” he says.
Cristián Gutiérrez-Ibáñez, a neurobiologist and bird researcher at the University of Alberta and lead author of the study, had initially planned to track down the origins of pedal dexterity in birds by observing and comparing multiple species in captivity.
However, COVID-19 disrupted those plans, rendering campus lockdowns and observations impossible. Consequently, he and his colleagues turned to the plethora of amateur and professional bird images available on social media and in online repositories. “People really love taking pictures of birds,” he notes.
After sifting through millions of media files, the team meticulously selected 3,725 images showcasing 1,054 species using their feet to manipulate objects, such as a spotted owlet biting off a mantis’ head and a hyacinth macaw nibbling on a palm nut. The researchers then plotted these behaviors onto evolutionary trees and discovered that the common ancestor of all these dexterous species was an early example of a clade of tree-dwelling birds. This clade, known as the Telluraves, includes parrots, falcons, owls, and songbirds. It began to diversify shortly after the mass extinction event that wiped out the dinosaurs 66 million years ago.
Because this clade of birds evolved to inhabit trees, the researchers speculate that its ancient ancestor may have initially developed primitive grasping skills in its feet to aid in holding onto tree branches, explains Gutiérrez-Ibáñez. The early Telluraves had long, backward-facing toes similar to many modern tree-dwelling bird species which could have functioned like opposable thumbs for grasping.
Additionally, their toe tendons could have provided a stronger grip. “The idea is that evolution into the trees facilitated the evolution of foot use, because now you have this sort of machinery that allows you to do it,” Gutiérrez-Ibáñez suggests. Later, certain bird species further honed their foot-use skills, developing abilities such as trapping, grasping, and lifting objects.
According to the study, this occurred at least 20 times in various lineages of the bird’s descendants. Parrots are an interesting exception—some scientists believe early parrots were hunters like owls and falcons, which may have bestowed their modern, fruit- and seed-eating descendants with adept grasping abilities, Gutiérrez-Ibáñez elaborates.