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Jays react angrily when shown a cup-and-balls-style magic trick in which their favourite snack is swapped for a less appealing one. Their responses show cognitive abilities that may come into play when they pilfer food caches hidden by other birds.

Eurasian jays (Garrulus glandarius) have impressive memories and show some capacity for imagining the beliefs and intentions of others, known as theory of mind. As such, and her colleagues at the University of Cambridge wondered whether jays would be sensitive to cognitive illusions designed to fool humans.

First, they tested six birds to find out which food each one preferred from a choice of worms, cheese and peanuts.

Then they showed the birds a version of the cups and balls magic trick, in which food was placed under one of two overturned cups. The cups had string handles so the birds could lift them.

The birds had seen a worm or cheese piece go into the cup, but in some cases the researchers swapped it for another type of food. If they expected to get their favourite food and found one they liked less, they were more likely to then look under the second cup, and in some cases rejected the food from the first cup completely. They were also slower to take food that wasn’t their favourite and were more likely to repeat picking up the cup where they expected their favourite to be.

Birds with a higher social rank were more likely to reject food they weren’t expecting to find, and tended to show stronger reactions, such as squawking and flying away. “They would get very cross,” says Schnell. “It’s hard not to anthropomorphise, but it’s like you could feel their frustration.”

This may be because dominant birds have more access to food, but it also chimes with what magicians say about humans, says Schnell. According to , a co-author of the study who performs magic, when magicians use “alpha” audience members as volunteers, they are more likely to react negatively to a trick. “It’s like they don’t want to be the butt of the joke,” says Schnell.

The birds’ reactions show an ability to imagine the immediate future, evaluate their expectations and use those evaluations to guide how they respond, she says.

This raises questions about whether jays use these capacities to make decisions about stealing other birds’ caches in the wild. “If they see another individual caching a seed, which is probably a mediocre treat, it might not bother. But if it sees this individual caching a worm, it might make a mental note to come back and steal that cache, using that memory and evaluating what that individual is expecting to find,” says Schnell.

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