Summary: Sex, Eggs, and Nests
Birds are strictly bisexual, with separate male and female organisms.
Hermaphroditism - both sexes in one individual - is a familiar condition
among reptiles, fish, invertebrates and plants, but is virtually unknown in
birds. Bird sex chromosomes evolved independently of mammalian sex
chromosomes, and differ in genetic structure. Females, rather than males,
are the sex with different sex chromosomes. A gene on the Z chromosome
determines sex in birds.
Birds typically lack external genitalia. The gonads - paired testes and usually a
single ovary - are located inside the body cavity on the surface of the kidneys
(Nieren). In male birds, sperm are mainly produced at night and are stored
at the end of the vas deferens (Samenleiter). Initially only a few millimetres
long, the testes can reach a 500 times larger size during the breeding season.
Testes of the Tree sparrow
(Feldsperling) in winter (A)
and during the breeding
season (B) (arrow: the