This young Snowy Egret stands on one leg because that’s enough. Why put out two legs when the air is so cold? Egrets’ legs have no feathers to keep them warm, so using both legs means losing heat twice as fast. We humans aren’t built very economically. We can’t draw one leg up close to our bellies to keep it warm while standing. Even if we could, we really can’t stand on one leg for very long, it’s uncomfortable and soon gets painful. A bird can keep it up all day. That’s odd when you look at the egret’s leg, how skinny it is, with hardly any muscle visible. You’d think it would need a fair amount of muscle to keep its balance. However, the main joint that we see in the photo can lock in place, without muscular effort to keep it steady. (That’s the ankle joint on a bird; the bird’s knee joint is up higher, hidden in the feathers.) And note that big foot; it provides a stable base. The bird can balance in calm weather just by flexing its toes a bit.
But note that the Burrowing Owls also like to stand on one leg, and their legs aren’t smooth and bare like the egret’s. They’re covered in short fuzzy feathers that look like they might provide some insulation. Here’s a detail of the First Owl on December 14 (photo right). That wasn’t an especially cold day and it’s unlikely that heat conservation was the key to the owl’s decision to tuck the other leg out of sight. It’s more likely that the owl just didn’t need that second leg for balancing and acted out of economy of effort. I noticed that when the owl became alert to possible danger from overhead, the second leg came out, as if to prepare the bird for flight or for a dash into a hideaway in the rocks, should that be necessary. But when it felt safe, one leg was enough.
Some scientists think that standing on one leg also serves a hunting bird, like this egret, as a kind of disguise. The one leg looks to a fish or a shrimp more like a plant, nothing to worry about. Possibly these fish and shrimp are nearsighted and can’t see what sort of branch and fruit sits higher up.