Perhaps no other pair of eyes in the animal kingdom is as recognizable as those of the chameleon. Not only do their big, protruding orbs rotate in all directions, but each eye can also independently scope its surroundings for food and predators, seeing two separate objects at the same time—a talent thought to be the chameleon’s alone.
But, as it turns out, Great-tailed Grackles might also have this skill. In a study published this past spring in Experimental Brain Research, scientists found that grackles were able to look simultaneously through each eye at different images. It’s still unknown whether the grackles focused on and truly registered the two images simultaneously—the distinction between “looking at” and actually “seeing” is an important one—but the findings raise that possibility, says Jessica Yorzinski, an animal behaviorist at Texas A&M University and author of the study
To test whether the Great-tailed Grackles could look at different objects, Yorzinski flashed images of human faces two at a time, with each image on a different monitor placed around the birds. Because songbirds perceive humans as threatening, the faces provoked strong visual reactions that Yorzinski recorded with video cameras.
By mounting the two monitors slightly behind and either above or below the birds, Yorzinski was also able to maximize their eye movements. When the images flashed, the grackles had to roll both eyeballs away from their beaks and either up or down to view the human faces simultaneously. After analyzing the video, she found that the birds frequently aimed one eyeball toward each picture at the same time.
The ability to look at two different objects simultaneously depends on how an animal’s eyes are oriented and whether they can move them independently. Human eyeballs, like those of owls and most mammals, sit on the front of our faces, locked together and focused on the same area. In contrast, Great-tailed Grackles and most other birds have lateral eyes positioned on the sides of their heads.
The benefits of such a skill could be many in the wild. “One eye could be looking at something over here, and the other eye could be looking at something over there,” Yorzinski says.“It's definitely a potentially really useful ability to be able to move each eye separately from the other one.” This is especially true if the birds are able to actually keep track of distinct objects. If a bird could forage on the ground using one eye while scouting the sky for predators with the other, that would be an evolutionary superpower akin to humans having eyes in the back of our heads.
While it’s important to specify the difference between what it means to look at an object and truly register it, it’s similarly worth noting that some birds with lateral eyes move them in different directions without them being truly independent. For instance, a 2009 study, published in the Journal of Experimental Biology, showed that Zebra Finch eyesight oddly seems to follow Isaac Newton’s third law of motion: For every action, there is an equal and opposite reaction. When one eye rotates up and forward to focus on an object, the other eye automatically rolls down and backward without focusing on anything.
It’s unclear whether the finch eye that rotates back is registering what it’s seeing, but the equal and opposite eye movement could prevent gaps from developing in the bird’s periphery, Hans-Joachim Bischof, a retired biologist who authored the Zebra Finch study, said in an email. Peripheral gaps could leave them more vulnerable to attack from predators.
So far, no other bird species is known to look at more than one object simultaneously, but a few other species, including pigeons and peafowls, can move their eyes independently. This indicates other birds may have visual systems that function similar to Great-tailed Grackles. “It wouldn’t be shocking to find that other species are able to do this as well,” Yorzinsky says. More testing is needed to understand how pervasive this trait is in birds.
“They could certainly be on a continuum,” Yorzinski says, which could help explain the differences between her findings and Bischof’s. She says it could also vary within species when they’re engaged in different activities such as flying, foraging, or singing.
As to the question of whether Great-tailed Grackle brains can truly register two things at once, Yorzinski says that it’s possible some birds have adapted to continuously scan their surroundings through both eyes. Or, they might rapidly shift attention between eyes, strobing awareness back and forth.
While questions remain, Yorzinski’s results offer a new glimpse into how birds see. Decades ago, researchers thought birds shifted their gazes primarily by moving their heads, and they largely ignored how eye motion contributes to bird vision. “It’s important to understand how species other than ourselves understand the world around them,” Yorzinski says. Learning that birds see ultraviolet light, she notes, has led to glass and sticker designs that protect them from deadly window strikes. “They're seeing in ways that we aren't, and maybe we can use that to help their conservation.”