Rooks team up to pull a food-laden tray into their enclosure
Pairs of rooks can co-operate to solve problems, scientists report.
An experiment revealed that the rooks would team up so they could reach a tray of food that was inaccessible to lone birds.
The researchers from the University of Cambridge were surprised to find that the birds performed as well as chimpanzees at the test.
The research is published in the journal Proceedings of the Royal Society B.
The researchers presented pairs of captive birds with a tray topped with tasty morsels of egg yolk and mealworm - however, it was placed just out of reach, outside of the birds' cage.
A single piece of string was threaded through two hooks on the tray, with each end left dangling 60cm (24in) apart, just inside the rooks' enclosure.
Psychologist Amanda Seed, the lead author of the paper, who is now based at the Max Planck Institute for Evolutionary Anthropology in Germany, said: "If just one bird pulled on one end of the string, it would slip out from the loops.
Rooks form monogamous relationships for life and live in colonies
"The question was would they work out, without any training, that they needed one bird to pull on one end of the string and another to pull on the other, simultaneously, to get to the food?"
The team, including Nicola Clayton and Nathan Emery, discovered that the eight pairs were happy to cooperate, with some pairs solving the task straight away, others taking a day or two to work out that team-work was the key to getting their nibbles.
Dr Seed told the BBC News website: "They performed remarkably well - as well as chimps when they were presented with the same test."
The team then gave the rooks another trial.
This time a single rook was presented with the same tray-string set-up, while its pair waited in a neighbouring cage linked by a one-way flap.
The idea was to see whether the rook would wait for the other rook to enter the enclosure so they could once again work together to reach the food.
Dr Seed said: "We found the birds just didn't wait."
The researchers believe that while rooks had the ability to cooperate, they may have failed to understand the importance and value of the act.
Dr Seed said: "The results suggest the rooks weren't using information about the efficacy of the partner: the need for the partner to solve that task."
However, chimps, when presented with the same scenario were happy to wait and team-up.
Dr Seed explained: "In terms of the cognitive mechanisms underpinning co-operation, there may be a difference between rooks and chimps.
"This could be because social groups of rooks and chimpanzees are structured differently.
"Chimpanzee society is a dynamic mix of cooperative and competitive relationships, whilst rook groups seem to be more stable."
Rooks (Corvus frugilegus) are members of the crow family. They live in colonies and form monogamous relationships for life.
The researchers are now keen to find out if other species of birds perform his kind of co-operative behaviour.
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