Mammals, like us, have 23 pairs of chromosomes. One pair comprise the sex chromosomes, one designated X and one Y. An individual with two X chromosomes, XX, is a female, and one with an XY combination is a male.
In birds it is reversed. The sex chromosomes in birds are designated Z and W, and the male is the ZZ and the female ZW. In most avian species the Z chromosome is a large chromosome, usually the fourth or fifth largest, and it contains almost all the known sex-linked genes. The number of chromosome pairs varies across bird species from 40 to 142 although most fall in the range of 78-84 pairs.
In male birds, the paired testes that produce sperm are located internally, unlike mammals whose testes descend outside the body in a scrotum to keep them a few degrees cooler than body temperature so that sperm develop normally. Avian testes, inside the body, are the same temperature as the bird body, about 40 degrees C but the extra heat is harmless. Testes vary in size over the year, growing considerably larger for the breeding season and in many species one testes, usually the left, is much bigger than the right. Females have only one ovary, the left one, although hawks and owls have two for a yet unknown reason.
Birds reproduce via internal fertilization, but most do not have a penis; only waterfowl and ratites such as the ostrich and emu possess one. The vast majority of birds transfer sperm from male to female via a “cloacal kiss.” A cloaca (derived from the Latin for drain), is the posterior opening that serves as the only opening for the digestive, reproductive, and urinary tract of birds. When the pair is ready for mating, the cloaca swells and protrudes a bit outside the body. During copulation, the male mounts the female from behind, arching his back, both sexes holding their tails to the side and moving the feathers around the cloaca so that the lips of the male’s and female’s cloacae can come into contact and the sperm transferred.
Of the birds that do have penises, there is a wide variation in phallic size and shape from the amazing Muscovy Duck’s eight-inch corkscrew penis that can be as long as its body when erect, to the ostrich’s bizarre-looking conical penis, that hangs to one side of its cloaca. Some birds have feathers on their penis, while others are covered in spines and small filaments. Male ducks, amazingly, grow a new penis every year. Female ducks have long twisted vaginas with spirals, but the vagina also has some dead-end pockets that they can utilize if they do not want to be fertilized by the male duck that is mounting them. The act of the cloacal kiss usually takes less than a second, but several kisses may occur.
Birds will usually mate several times for about a week to increase the chances of successful insemination and to maintain the pair bond. Whether the male leaves or stays to help the female in nest building, incubation of eggs, and raising young depends on the species and, over 10,000 species, there is a lot of variation.