The majority of these animals are passerines, and have a backward-facing toe.
The majority of these animals are passerines, and have a backward-facing toe. These organisms evolved independently of mammals, but they are still endothermic vertebrates with four-chambered hearts. Unlike their ancestor (*) Archaeopteryx, these animals lack teeth. Hard eggs and light skeletons are characteristics of, for ten points, what class of feathered animals that includes the flightless ostrich?
The majority of these animals are passerines, and have a backward-facing toe.
birds (accept Aves)