Male birds have two testes that produce sperm, and most female birds usually have only one ovary that produces eggs. A bird's testes or ovary greatly increase in size during the breeding season. Their small size during the rest of the year lightens the load for flight. The cloaca is the outlet for eggs or sperm.

After mating, the female lays a number of eggs in a clutch. Different species lay different numbers - from one to 20 eggs. Birds that lay many eggs are generally those that live in higher latitudes, lay small eggs, nest in holes rather than in the open, and give birth to well-developed young. Other factors determine clutch size and some, like food availability, may change from year to year.

Eggs range in size from 13 inches (extinct elephant birds of Madagascar) to only 1/3 inch long (hummingbirds). Smaller birds lay large eggs in proportion to their body weight. Hummingbirds lay eggs that are 15 % of their body weight, while ostriches lay eggs that are only 2% of their body weight. Flightless kiwis lay only one huge egg that can be 25% of their body weight.

Egg shape and color often reflect the nest environment. Cliff-nesting birds lay eggs that are pointed at one end; when bumped, they roll in a tight circle and not off the cliff.

Nearly all birds sit on their eggs to keep them at the best temperature for growth. This is called incubation. Either one sex or both sexes incubate. Eggs are turned so they warm evenIy and develop properly. Many birds develop a brood patch, an exposed patch of skin with many blood vessels to provide extra heat for the eggs during nesting. Depending on the species, incubation may last from 10 days to more than two-and a-half months.

Some newly hatched chicks are precocial. They hatch covered with down, their eyes open, and can immediately run or swim to escape predators. Most chicks are altricial. They are blind, helpless, naked, and completely dependent on their parents for food. Females of altricial young spend less energy by laying a smaller egg and incubating it a shorter time than precocial mothers; but they spend more energy providing the greater care altricial young need after hatching.