Although mammals, the S'kytri hatched from eggs. Their wings, even though leathery looking, actually had a fine down of invisible feathers. Their skeletons were hollow, much like a bird's, which allowed them easier flight. Female S'kytri were born with green skin while males were born with blue. Any egg that was perceived to have a female with blue skin or a male with green was immediately killed. Their haircolor ranged from moonlight blond to jet black, and their eyes from dark blue to light silver and every other color in between.[1]
Though continuity specifies that only blue or green-skinned S'kytri are ever born, the Ultimate Alien Anthology features an image of a red-skinned female.