For the New Caledonian crow, birdbrain is a misnomer: These members of the corvid family have proved their problem-solving and tool-wielding abilities again and again. The birds may have yet another impressive cognitive capacity, a new study suggests: causal reasoning. The ability to link an event with the mechanism that caused it, even if that mechanism is hidden, is the basis of modern science—and our most basic knowledge of the world around us. If New Caledonian crows are capable of causal reasoning as well, we can better trace and understand the evolution of this ability.
Researchers described how they tested the crows’ reasoning in the Proceedings of the National Academy of Sciences. First, they let eight New Caledonian crows figure out how to use a tool to remove food from a hole in a box. Then the experiment began. The crows watched as a human entered their enclosure and stood by the box. But this motionless figure, eyes closed and hands in a neutral position, did not pose as much of a threat as the blue cloth hanging over one side of the enclosure. Through a gap in this bird blind, a stick emerged and poked at the hole in the box 15 times. Finally, the stick stopped moving and the non-threatening observer left the aviary.
Because the birds had to turn their heads away from the bird blind—where the probing stick had emerged—in order to nab a treat from the box, they were understandably cautious after the stick withdrew. Although they gradually went back to extracting food, they first inspected the bird blind and abandoned some preliminary probes of the food box. However, the birds were less wary and exhibited less testing behavior when they saw a second human enter the bird blind before the stick started moving, and leave the blind after the stick’s motion stopped. They recognized that the hidden human was the cause of the moving stick (even though, for consistency with the single-human situation, the stick was actually under the control of an experimenter outside the enclosure).
Understanding that a human was moving the stick, and that it would stop moving in the human’s absence, is more than a knowledge of cause and effect. Because the experimenter was invisible behind the bird blind, the crows did not actually see anyone moving the stick. And yet they could still infer that the human presence was responsible for the stick’s motion, which indicates that they can recognize even a hidden causal agent. This is a very impressive ability—until now, the authors write, “Although [research had suggested] animals can reason about the outcomes of accidental interventions, only humans have been shown to make inferences about hidden causal mechanisms.”
Image courtesy of John Gerrard Keulemans / Wikimedia Commons