In an experiment, researchers found that crows were more likely to forage when they could attribute changes in their environment to a human presence.

This behaviour may suggest "complex cognition", according to a study published in the Proceedings of the National Academy of Sciences.

Until now the ability to make inferences based on causes has been attributed to humans but not animals.

The study was a collaboration between researchers from the University of Auckland, New Zealand, the University of Cambridge, UK and the University of Vienna, Austria.

In their experiment eight wild crows used tools to remove food from a box.

Inside the enclosure there was a stick and the crows were tested in two separate series of events that both involved the stick moving.

In one instance a human entered the hide and the stick moved. In the other, the stick still moved but no human entered.

On the occasions when no human was observed entering the hide, the crows abandoned their efforts to probe for food using a tool more frequently than they did when a human had been observed.