In the male bird, sperm is produced in two testes that lie anterior to the kidneys. Sperm passes through small tubes called vasa deferentia (singular, vas deferens) into the male's cloaca. During mating, the male presses his cloaca to the female's cloaca and releases sperm. Most females have a single ovary located on the left side of the body. The ovary releases eggs into a long funnel-shaped oviduct, where the eggs are fertilized by sperm. Fertilized eggs move down the oviduct, where they are encased in a protective covering and a shell. The egg passes out of the oviduct and into the cloaca. From the cloaca, it is expelled from the bird.

Nest Building and Parental Care

Birds usually lay their eggs in a nest. Nests hold the eggs, conceal young birds from predators, provide shelter from the elements, and sometimes serve to attract a mate. Most birds build nests in sheltered, well-hidden spots—ranging from holes in the ground to treetops. Woodpeckers, for example, nest in a hole they have drilled in a tree. Orioles suspend their nests from branches, well beyond the reach of predators. And barn swallows build a saucer of mud on the beam of a building. Birds construct their nests from almost any available material. Twigs, bark, grasses, feathers, and mud are common materials used.

One or both parents warm, or incubate, the eggs by sitting on them and covering them with a thickened, featherless patch of skin on the abdomen called a brood patch. Once the eggs hatch, the young usually receive extensive parental care.

Birds have two general patterns of rearing young. Some birds lay many eggs and incubate them for long periods. These birds hatch precocial (pree-KOH-shuhl) young, which can walk, swim, and feed themselves as soon as they hatch. Ducks, quail, chickens, and other ground-nesting species produce precocial offspring. Other birds lay only a few eggs that hatch quickly and produce altricial (al-TRISH-uhl) young, which are blind, naked, and helpless, as shown in Figure 42-7. These young depend on both parents for several weeks. The young of woodpeckers, hawks, pigeons, parrots, warblers, and many aquatic birds are altricial.

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