House sparrow
Eggs and young

Clutches usually comprise four or five eggs, though numbers from one to 10 have been recorded. At least two clutches are usually laid, and up to seven a year may be laid in the tropics or four a year in temperate latitudes. When fewer clutches are laid in a year, especially at higher latitudes, the number of eggs per clutch is greater. Central Asian house sparrows, which migrate and have only one clutch a year, average 6.5 eggs in a clutch. Clutch size is also affected by environmental and seasonal conditions, female age, and breeding density.

Some intraspecific brood parasitism occurs, and instances of unusually large numbers of eggs in a nest may be the result of females laying eggs in the nests of their neighbours. Such foreign eggs are sometimes recognised and ejected by females. The house sparrow is a victim of interspecific brood parasites, but only rarely, since it usually uses nests in holes too small for parasites to enter, and it feeds its young foods unsuitable for young parasites. In turn, the house sparrow has once been recorded as a brood parasite of the American cliff swallow.

The eggs are white, bluish white, or greenish white, spotted with brown or grey. Subelliptical in shape, they range from 20 to 22 mm (0.79 to 0.87 in) in length and 14 to 16 mm (0.55 to 0.63 in) in width, have an average mass of 2.9 g (0.10 oz), and an average surface area of 9.18 cm (1.423 in). Eggs from the tropical subspecies are distinctly smaller. Eggs begin to develop with the deposition of yolk in the ovary a few days before ovulation. In the day between ovulation and laying, egg white forms, followed by eggshell. Eggs laid later in a clutch are larger, as are those laid by larger females, and egg size is hereditary. Eggs decrease slightly in size from laying to hatching. The yolk comprises 25% of the egg, the egg white 68%, and the shell 7%. Eggs are watery, being 79% liquid, and otherwise mostly protein.

The female develops a brood patch of bare skin and plays the main part in incubating the eggs. The male helps, but can only cover the eggs rather than truly incubate them. The female spends the night incubating during this period, while the male roosts near the nest. Eggs hatch at the same time, after a short incubation period lasting 11–14 days, and exceptionally for as many as 17 or as few as 9. The length of the incubation period decreases as ambient temperature increases later in the breeding season.

Young house sparrows remain in the nest for 11 to 23 days, normally 14 to 16 days. During this time, they are fed by both parents. As newly hatched house sparrows do not have sufficient insulation, they are brooded for a few days, or longer in cold conditions. The parents swallow the droppings produced by the hatchlings during the first few days; later, the droppings are moved up to 20 m (66 ft) away from the nest.

The chicks' eyes open after about 4 days and, at an age of about 8 days, the young birds get their first down. If both parents perish, the ensuing intensive begging sounds of the young often attract replacement parents which feed them until they can sustain themselves. All the young in the nest leave it during the same period of a few hours. At this stage, they are normally able to fly. They start feeding themselves partly after 1 or 2 days, and sustain themselves completely after 7 to 10 days, 14 at the latest.

House sparrows in natural small populations, as can occur on islands, exhibit inbreeding depression. Inbreeding depression is manifested as lower survival probability and production of fewer offspring, and can occur as a result of the expression of deleterious recessive alleles. However sparrows in such populations do not appear to avoid inbreeding.
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