It’s energy efficient not because of physical effort/effort to stand, but because birds spend energy to warm them body. Since most of their is covered in feathers, they are extremely well insulated. Their legs and feet however are exposed so they most the majority of their body heat through that exposure. It’s sort of like how you might stick one leg out from under the covers of your bed when you’re hot, but both legs out are too cold. So you only stick one leg out and that’s a nice comfortable medium place.
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