Back to feet: Smaller birds with shorter legs, particularly species favoring trees, hop on land. Birds with longer legs are more likely to walk. Some, like American robins, do both, depending on need.

A C.T. scan of the foot of a 99 million-year-old bird whose feathers, not visible on the scan, were preserved in amber. Credit... Lida Xing et al.

A new analysis of Archaeopteryx, the earliest-known birdlike animal, shows it had feet like dinosaurs — which supports the view that today's birds descended from ancient reptiles.

A bird with frozen feet is generally not among them. "Very very rarely, I have seen it," she tells Treehugger. And only on those occasions that a flash freeze caught them by surprise.