Crows seem to be able to use causal reasoning to solve a problem, a feat previously undocumented in any other non-human animal, including chimps.
Alex Taylor at the University of Auckland, New Zealand, and his team presented six New Caledonian crows with a series of "trap-tube" tests.
A choice morsel of food was placed in a horizontal Perspex tube, which also featured two round holes in the underside, with Perspex traps below.
For most of the tests, one of the holes was sealed, so the food could be dragged across it with a stick and out of the tube to be eaten. The other hole was left open, trapping the food if the crows moved it the wrong way.
Three of the crows solved the task consistently, even after the team modified the appearance of the equipment. This suggested that these crows weren't using arbitrary features – such as the colour of the rim of a hole – to guide their behaviour. Instead they seemed to understand that if they dragged food across a hole, they would lose it.
Not-so great apes
To investigate further, the team presented the crows with a wooden table, divided into two compartments. A treat was at the end of each compartment, but in one, it was positioned behind a rectangular trap hole. To get the snack, the crow had to consistently choose to retrieve food from the compartment without the hole.
A recent study of great apes found they could not transfer success at the trap-tube to success at the trap-table. The three crows could, however.
"They seem to have some kind of concept of a hole that isn't tied to purely visual features, and they can use this concept to figure out the novel problem," Taylor says. "This is the most conclusive evidence to date for causal reasoning in an animal."
Three of the crows did fail at both tasks, however. The team plans further work to investigate why.