Bird Literally Weighs Its Food Options

Mexican Jays compare peanuts to determine which one has the most meat inside before choosing one for a meal. Karen Hopkin reports.

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If you’ve ever been to an all-you-can-eat buffet, you know how important it is to . You don’t want to fill up on salad when so many await. It seems some birds also weigh their mealtime choices—literally. A study finds that Mexican Jays pick up and shake peanuts to assess their relative heft before choosing one. That report is served up in the . [Piotr G. Jablonski et al, ]

Foods that hide their edible bits on the inside present a challenge to hungry diners. How can you or which shells harbor the biggest nuts? We humans knock on melons or squeeze avocados. But how do other species select the highest quality snacks?

To find out how the jays do it, researchers fiddled with their feed. First they doctored peanuts so that some contained three nuts while others had none. When they offered these pods to some jays, the birds turned their beaks up at the empty shells and instead chose those that were full. And when the jays were allowed to choose between normal peanuts and those that weighed just one gram more, because the researchers had stuffed them with clay, the birds again went for the heavier meal.

Videos revealed that the jays shake the nuts before selecting one, which apparently lets them gauge the nut’s mass and perhaps also listen for the rattle of a well-packed shell. Pretty clever for a bird brain.

—Karen Hopkin

Mexican Jay sounds courtesy of Cornell Laboratory of Ornithology