The testes of a male bird increase significantly during the breeding season. This dramatic change seems to occur mostly in temperate and migratory birds. Its unclear if tropical birds really do the same thing or not.
The testes of birds are not much different than that of other vertebrate animals. The sperm are also very similar to other vertebrates. The acrosome is at the tip (for penetrating the egg). The midpeice, which is loaded with mitochondria is used for power. Passerine birds, however, do have a slightly different sperm morphology.
The cloacal protuberance is an extension from the abdominal body wall. This protuberance indicates a male. It’s the most common male external reproductive feature. The cloaca is where sperm, urine, feaces etc leaves the body.
For copulation, the male has to get his cloaca close to the tip of the cloaca of a female bird. The male protuberance is intermittent in the fact that it will change its length during the breeding season.
Roosters havea phallus or a penis. It is relatively short, in any case.
Ostrich males have a penis that’s about 20 centimeters long. Some birds of prey and galliform birds also have an intermittent organ that works the same as a penis.
Female birds also have an increase in the size of their reproductive organs. In birds, it's generally the left ovary that is active. The ovaries look very similar to a bunch of grapes.
Each one of the circular ‘grape-like’ structures are called follicles that can be atretic or mature eggs. Inside each follicle is the ovum (germ cell), plus the yolk. An atrectic follicle is one that has started to develop but then aborted.
By Rob Nelson
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