In birds, the cloaca is a multi-purpose opening: waste is expelled through it, birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets.
Anatomy (from the Greek ἀνατέμνω – anatemnō, "I cut up, cut open" from ἀνά – ana, "on, upon", and τέμνω – temnō, "I cut") is the study of the body plan of animals. In some of its facets, anatomy is closely related to embryology, comparative anatomy and comparative embryology, through common roots in evolution. Human anatomy is important in medicine.
The discipline of anatomy is subdivided into gross (or macroscopic) anatomy and microscopic anatomy. Gross anatomy is the study of structures that can, when suitably presented or dissected, be seen by unaided vision with the naked eye. Microscopic anatomy is the study of structures on a microscopic scale, including histology (the study of tissues) and cytology (the study of cells).
Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has led to evolution of a specially adapted digestive system. These anatomical specializations have earned birds their own class in the vertebrate phylum.
Fertilization which takes place inside the female body is called internal fertilization. Internal fertilization in animals is done through copulation, which involves the insertion of the penis or other intromittent organ into the vagina (in most mammals) or to the cloaca in monotremes, most reptiles, some birds, the amphibian tailed frog and some fish, the disappeared dinosaurs, as well as in other non-vertebrate animals. In some birds and in the tuatara, that don't have an intromittent organ, the internal fertilization is done by the means of the cloacal kiss, which consists in that the two animals touch their cloacae together in order to transfer the sperm of the male to the female.
At some point, the growing egg or offspring must be expelled. There are three general ways of doing this: