Dozens of times a second, a hummingbird's tongue flicks in and out of its long beak, lapping nectar out of the flower that it hovers over. For centuries, biologists have puzzled over the process: How does the bird's unique tongue—narrow, forked at the end, and covered in grooves—get liquid into its mouth? Scientists once thought the explanation lay in capillary action, the phenomenon by which liquid flows up thin tubes through surface tension alone. But last year researchers found that the tiny grooves on the humming bird's tongue actually close when coming into contact with liquid, creating a fluid trap. The findings put to rest the capillary action theory, but not for good. Now, new high-speed video of ruby-throated hummingbirds (Archilochus colubris) shows that both hypotheses are right: When the bird is drinking from a flower with deep nectar reservoirs and the tongue is entirely submerged, the fluid trap works optimally. But in a shallow pool of nectar, the fluid trap garners little nectar with each extension of the tongue, since it only traps fluid that it is submerged in. In this case, capillary action draws liquid up the grooves to save the bird energy—it doesn't have to stick its tongue in and out as many times. The results, which appear online today in the Proceedings of the Royal Society B, suggest that the hummingbird has evolved to feed from multiple flower types  and not rely on a single tongue mechanism that limits potential food sources.
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