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An examination of perched Willow Warblers Phylloscopus trochilus singing on one leg

Territory-holding male Willow Warblers Phylloscopus trochilus stood on one leg when they were singing at song posts for extended periods of time. The other leg was tucked in the breast plumage and not visible. The length of time spent on a twig was positively correlated to the use of unipedal stance. Observed birds spent longer pose time perched on one leg than on two legs. The right leg and the left leg were used at similar frequencies. The reason for this behaviour might be to save energy, by reducing heat loss through the skin of the tucked leg.