How do you identify an optimistic crow?

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The native crows on the Pacific island of New Caledonia are adept at making and using tools to extract morsels of food from difficult-to-reach crevices. Biologist Cody McCoy wondered whether using a tool – effectively solving a puzzle – made crows more optimistic. If a human being is feeling optimistic they might describe a glass of water as half-full rather than half- empty. But how do you work out if a crow’s feeling optimistic? (Image: Crow holding stick, New Caledonia Credit: Dr. Dakota McCoy)