Birds bones are light weight for flight. Birds have shelled eggs and so must have internal fertilization the egg muct be fertilized before the hen s reproductive tract secretes an eggshell. Generally, mating is accompanied by an elaborate courtship ritual. Mammals have a diaphragm to aid in respiration. They are endothermic.
Most mammals bear live young.
These mammals lays eggs like reptiles, but do have fur and milk. They have one posterior opening for their digestive, urinary, and genital tracts, hence the subclass name. The young are born as very immature embryos and must crawl to their mother s pouch to continue their development. Because of continental drift, there is a wide variety of marsupials in Australia, yet few elsewhere on Earth. In this taxon, young complete embryonic development within the mother s uterus and are nourished across a placenta .