Crows are more clever than you might think. They use tools to get food, can plan ahead and even can remember human faces.
Zoologists are fascinated with complex behaviors shaped by evolution, such as nest building. But occasionally, they are stumped by what University of Oxford zoologist Alex Kacelnik calls “spooky” behaviors for which there is no ready explanation.
Solving a novel problem that an animal doesn’t usually encounter in nature is clearly one of them.
Kacelnik has been studying New Caledonian crows, a corvid species from the South Pacific that has shown remarkable talent for using tools. One crow in particular, named Betty, surprised researchers when she refashioned a tool to create a hook. (You may have seen her in a popular documentary on PBS’ “Nature” series).
“Crows and parrots have shown in the last 10 years or so that they can do many such things, surprising those who expected that only close relatives of humans such as other primates or mammals would show them,” said Kacelnik.
All crows are unusual in another important way that piqued Kacelnik’s interest. They have a very wide field of binocular vision – a 60-degree arc where the fields of each eye overlap. That’s more than double the binocular field of vision for pigeons, and within the range of human binocular sight.
Kacelnik and others at the University of Oxford wondered if binocular vision might help crows manipulate tools to dig out their prey in the same way that such sight helps humans. They were curious how tool use may have influenced vision over the course of evolution.
“Tool use plays an enormous role in the economy of New Caledonian crows, perhaps the biggest role in any species other than humans,” Kacelnik said. “It is also known to be a feature of the species and to have heritable properties: Baby crows show tool use before seeing anybody else doing it. Their unusually large binocular field toward the front is likely to relate to this, but we have no firm evidence as yet.”
A study Kacelnik and his team published online Dec. 4 in the journal Current Biology offers some surprising answers to at least part of that mystery.
New Caledonian crows have a tendency to choose one side of their beak over another to wield a stick – somewhat like right- and left-handedness among humans, the study found. The birds choose based on which eye is stronger, or dominant, because it turns out that monocular vision may have a larger role in helping crows extract larvae from their tubular dens, according to the study.
The researchers tested for eye dominance first, and found four of nine birds were right-eye biased, and five left-biased. The birds also tended to slant the working end of the stick toward their non-dominant eye. The split was three to six, which is close enough, statistically.
To make sense of this preference, researchers modeled how the depth and diameter of a larva burrow affect lines of sight for each eye as the birds insert a stick into the tube. That let them figure out the various scenarios in which birds would use binocular or monocular sight.
The geometry showed that when it first picks up the stick, the bird can see its tip binocularly. And the crows can see that way for a while as they approach the hole and place the stick inside, the study found.
To thread the stick, however, the bird turns its head toward the dominant eye side. The dominant eye then peers across the beak, straight down the hole. The non-dominant eye has no such obstruction, but it also is peering at the hole from an oblique angle, and eventually can’t see the tip as it moves deeper. Monocular vision with the dominant eye can track the tip.
The geometry is somewhat analogous to angling the body to shoulder a rifle, and training the dominant eye down the barrel.
When the researchers measured what the birds actually did to extract prey, they found that the majority of successful stabs occurred in scenarios favoring monocular vision by the dominant side.
Eye dominance, in other words, appears to drive the way the birds use tools. Because eye dominance has been shown to be random across the species, so is tool preference.
That’s not so for humans, 90 percent of whom are right-handed, even though two-thirds are right-eye dominant, the authors note.
But humans also don’t manipulate tools with their mouths. Their hands also are distant from the head, and move independently of it.
Neither condition holds for birds, which have to clutch a spear shaft in their mouth, then thread its tip down a tube.