Dropping stones

“I had always wanted to see if there was a way to test what the crow did in Aesop’s fable,” explains Nathan Emery, University of London.

As the 2000-year-old story goes, the crow filled the bucket of water with stones until the level became high enough for him to quench his thirst.

A number of corvids have been found to use tools in the wild, and New Caledonian crows appear to understand the functional properties of tools and solve complex physical problems via causal and analogical reasoning. A 1980s study tells how a rook plugged a hole in its aviary to allow a pool of water to form.

Emery’s experiment includes four captive rooks, presenting them with clear tubes partly filled with water with a bug floating on top, and piles of stones. The tube contained the larvae of a wax moth–the birds’ favorite food–floating near the bottom, just beyond the reach of the rooks’ beaks.

The researchers then placed a small pile of stones next to the tube; in some of the experiments, these varied in size, so the birds had a choice of using either large or small stones. The amount of water in the tube also varied, requiring that the birds drop between one and seven rocks in order to get the prized worm.

Video footage is astonishing: it shows the rook first assessing the water level by peering at the tube from above and from the side, before picking up and dropping the stones into the water. Within a couple of trials the birds had figured out how many stones they needed to bring the bug within reach.

The experiment is a further demonstration of convergent cognitive evolution between the primate and corvid lineages, with both groups having a generalized understanding of the physical world. The rooks are “clearly combining some sort of understanding of the task with an understanding of the tool and are able to solve the task so quickly.” At the very least, adds Emery, the experiments demonstrate the rooks’ talent for “innovation, because they are adapting their previous experience with stones and tubes [in other experiments] to a new problem.”