Some Hooded crows (Corvus cornix) understand how a loose string works.

In a loose-string task an out-of-reach tray baited with food can only be retrieved by simultaneously pulling on both ends of a string threaded through the loops on the tray. This task is used to assess an animal's ability to cooperate, with each animal only having access to one end of the string. Some studies use the loose-string task in a pre-training phase, during which animals are individually taught to pull both ends of the string. Usually, no additional tests are conducted to determine whether the animals have understood how the loose string works. It is conceivable that a lack of knowledge of the causal basis of the loose-string task could make it more challenging to grasp how the partner can assist with it. Here, we tested whether Hooded crows could acquire some knowledge of the causal basis of the loose-string task. Prior to the critical test (Phase 3), the birds were presented with two different tasks (Phase 1 and Phase 2) to allow them to acquire some knowledge of the causal basis of the task. The results may indicate that, as a consequence of the experience gained, some crows may have begun to understand how the loose string works.