Birds normally reproduce sexually by the fusion of male and female gametes— spermatogonia and oogonia—produced by differentiated testes and ovaries. Fertilization is internal and all birds are oviparous. Eggs are laid at the gastrula stage, and almost all embryonic development occurs outside the female’s body. The egg is composed of layers that nourish or protect the embryo and are usually laid so that the young hatch when seasonally variable conditions are most favorable. The assumption of parental duties by the adults enables embryonic development to proceed optimally and contributes to the survival of the neonates. Parental investment is a primary breeding strategy of birds, to maximize the success of limited matings. These are usually only between socially bonded partners, restricted seasonally.
KeywordsLuteinizing Hormone Sertoli Cell Prolactin Secretion Gonadal Steroid Nest Building
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