The first reason bird legs look strangely thin is that the rest of their body is mostly covered by feathers, which can dramatically magnify its apparent size, says Julia Clarke, a paleontologist who studies bird evolution at the University of Texas at Austin. Birds’ relatively meatier thighs are tucked underneath their feathers, and most birds have scaly lower legs, Clarke says, which makes these limbs look oddly small and naked. Birds that have feathers down to their toes—such as owls and some domestic chickens—appear to have thicker shins than their bare-legged cousins.

But it’s not just a visual illusion: bird legs have some genuine differences from legs of other living animals.