Some birds, such as chickens, are continuous breeders, reproducing throughout the year. Other birds such as parrots are opportunisitic breeders, breeding during certain cycles or when favorable conditions exist.
Spring heralds the onset of reproduction in many avian species. Reproduction occurs most successfully when the environmental conditions are favorable for the survival of offspring and food is plentiful.
Day length or photoperiod plays an important role in birds that are seasonal reproducers. Environmental light stimulates neural receptors which, in conjunction with an internal circadian cycle, enables the bird to respond to the most favorable time for reproduction. As day length increases in the winter and spring, the ovaries and testes of most temperate species increase in size and undergo development.
In addition to day length, temperature and humidity influence reproduction in many free ranging species. In areas where the climate is stable and dry and day length is constant, rainfall may trigger reproductive behavior.
In the budgie male, vocalization triggers reproduction by stimulating ovarian development and ovulation.
In certain species, such as cockatiels, the presence of a mate is necessary to ensure nesting behavior.
In colony breeding species, such as budgies, the presence of other breeding birds is a reproductive stimulus. Auditory as well as visual contact stimulates reproduction.
The availability of a suitable nest and nesting material plays an important stimulus for breeding in some species such as finches and cockatiels.
Biologic clocks, known as circannua cycles, control the release of hormones that regulate reproduction, metabolism and behavior.
As previously mentioned, photoperiod plays an important role in many psittacine species. Light stimulates a part of the brain – the hypothalamus – to produce “releasing factors”. These releasing factors stimulate the anterior pituitary to secrete hormones known as genadotrophins. Follicle stimulating hormone (FSH) and Leutenizing Hormone (LH) are tow genadotrophins produced by the anterior pituitary which affect the ovaries and testes. FSH, and to a lessor degree LH, are responsible for normal ovarian follicular growth. As the follicles increase in size, they produce increasing amounts of estrogen and progesterone. Proogesterone acts on LH – which then triggers ovulation. Once ovulation occurs, progesterone secretion decreases rapidly. Estrogen is responsible for numerous female secondary sexual characteristics.
In the male, FSH initiates seasonal growth and development of the seminiferous tubules in the testes and spermatogenisis. LH promotes the production of testosterone, the male hormone responsible for the production of secondary sexual characterisitics and behavior.
Females may be determinate or indeterminate egg layers. Budgies are an example of determinate layers – laying a set number of eggs in their clutch, not replacing eggs that are removed or destroyed. Many of the large psittacines, chickens and ducks are indeterminate layers, replacing eggs that are lost or removed from their clutch. Aviculturists often remove eggs from nest boxes and artificially incubate them – thus stimulating “double clutching”.
Birds differ from mammals in that the female is heterozygous (ZW) rather than the male. The sex of the embryo is determined by the female prior to ovulation and not at fertilization.
The age at which sexual maturity is reached varies among species. Macaws, cockatoos and amazons reach sexual maturity at three to six years of age, while tiny finches can reproduce as young as two months of age.