Animals that move backwards usually require a lot of energy so, when biologist Nir Sapir observed agile hummingbirds visiting a feeder on his balcony, he was surprised to observe that hummingbirds execute this maneuver routinely. Wondering how hummingbirds perform the feat, he analyzed their flight and the amount of oxygen they consume and found that, for hummingbirds, reversing is much cheaper than hovering flight and no more costly than flying forward.

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