English: [video only-no audio] A garden warbler displays Zugunruhe in the lab. Zugunruhe, German for nocturnal migratory restlessness, occurs every spring and fall, and is powerful evidence of the internal clock that guides the timing of migration. "Biological clocks are found in any animal, from worms and flies up to humans," says Paul Bartell, an assistant professor of avian biology at Penn State. Working in concert with daylight and other environmental cues, these innate regulators control not just patterns of sleeping and eating, but hormone releases and many other cell processes as well. In songbirds, they dictate the delicate timing of migration.