Some embryos are formed in birds spontaneously, which are called wind eggs and 'zephyria' by some; these occur in birds which are not given to flight nor rapine but which produce many young, for these birds have much residual matter, whereas in the birds of prey such secretion is diverted to the wings and feathers, while the body is small and dry and hot; and the menstrual secretion and the semen are residues.