Birds and mammals are both warm-blooded animals with separate sexes determined by genetics, spinal cords, four-chambered hearts, four limbs and well-developed bony skeletons. Both groups include species that can fly and those that cannot, and both groups include species that lay eggs.
Birds and mammals also bear many differences. While birds and mammals are the only vertebrate groups with flying members, the majority of mammal species cannot fly, while the majority of birds can. All birds lay eggs, while only a few mammal species do, and mammal eggs are leathery, unlike the hard-shelled bird eggs. Mammals have solid bones, while most birds have hollow bones to reduce their weight. Most mammals have well-developed teeth, while birds have no teeth as adults.
All female mammals create milk to feed their young, and birds lack this ability. Birds have the ability to see into the ultraviolet spectrum and generally have well-developed vision and poor senses of smell. By contrast, smell is a key sense for many mammals. All birds, except ostriches, mix their liquid and solid waste when eliminating.
Most species of mammals either have one male mating with several females or males and females mating promiscuously. Birds are often monogamous, at least during the breeding season, sometimes for life.