The budgerigar (Melopsittacus undulatus) , also known as the common parakeet or shell parakeet and usually informally nicknamed the budgie, is a small, long-tailed, seed-eating parrot. Budgerigars are the only species in the Australian genus Melopsittacus and are found wild throughout the drier parts of Australia where the species has survived harsh inland conditions for the last five million years. Budgerigars are naturally green and yellow with black, scalloped markings on the nape, back and wings, but have been bred in captivity with colouring in blues, whites, yellows, greys and even with small crests. Budgerigars are popular pets around the world due to their small size, low cost and ability to mimic human speech. The origin of the budgerigar's name is unclear. The species was first recorded in 1805, and today is the third most popular pet in the world, after the domesticated dog and cat. The budgerigar is closely related to the lories and the fig parrots. They are one of the parakeet species, a non-taxonomical term that refers to any of a number of small parrots with long, flat and tapered tails. In both captivity and the wild, budgerigars breed opportunistically and in pairs.
Breeding in the wild generally takes place between June and September in northern Australia and between August and January in the south, although budgerigars are opportunistic breeders and respond to rains when grass seeds become most abundant. They show signs of affection to their flockmates by preening or feeding one another. Budgerigars feed one another by eating the seeds themselves, and then regurgitating it into their flockmate's mouth. Populations in some areas have increased as a result of increased water availability at farms. Nests are made in holes in trees, fence posts or logs lying on the ground; the four to six eggs are incubated for 18–21 days, with the young fledging about 30 days after hatching. In the wild, virtually all parrot species require a hollow tree or a hollow log as a nest site. Because of this natural behaviour, budgerigars most easily breed in captivity when provided with a reasonable-sized nest box. The eggs are typically one to two centimetres long and are pearl white without any colouration if fertile. Female budgerigars can lay eggs without a male partner, but these unfertilised eggs will not hatch. Females normally have a whitish tan cere; however, when the female is laying eggs, her cere turns a crusty brown colour. Certain female budgies may always keep a whitish tan cere or always keep a crusty brown cere regardless of breeding condition. A female budgerigar will lay her eggs on alternate days. After the first one, there is usually a two-day gap until the next. She will usually lay between four and eight eggs, which she will incubate (usually starting after laying her second or third) for about 21 days each. Females only leave their nests for very quick defecations, stretches and quick meals once they have begun incubating and are by then almost exclusively fed by their mate (usually at the nest's entrance). Females will not allow a male to enter the nest, unless he forces his way inside. Depending on the clutch size and the beginning of incubation, the age difference between the first and last hatchling can be anywhere from 9 to 16 days. At times, the parents may begin eating their own eggs due to feeling insecure in the nest box. Sometimes, budgerigars (mainly males) are not interested in the opposite sex, and will not reproduce with them; a flock setting—several pairs housed where they can see and hear each other—is necessary to stimulate breeding.