Passerine birds are the most diverse and abundant group of birds. Containing the majority of living bird species, they inhabit every continent on Earth except Antarctica. What unites them is their tendency to perch and a body form to match. Passerine birds have three toes forward and one backward, which allows them to hold onto perches. Whether perching on branches, cliffs, rocks, fences, or other surfaces, rough skin on the bottom of the foot provides traction. A tendon on the back of each leg tightens to bend the bird's foot so that it curls around the perch. Many passerines spend the night on perches without falling off. Stiff tail feathers help them balance on vertical perches such as tree trunks. Their ability to perch makes a huge range of habitats available to them. A robin in New York City and a bird of paradise in a forest in New Guinea are both passerines.

Many male birds use brightly colored feathers to show off to females during the breeding season. Most males set up territories containing important resources (food, nesting materials), and try to attract females while keeping males out. Courtship can be an elaborate affair, with displays by the male and sometimes by females, depending on the species. After mating, all birds lay eggs. As the egg develops, the yolk gets covered with layers of egg-white (to feed the embryo), shell (for protection), and pigment (for color). Eggs must stay at the right temperature for development. Most birds incubate their eggs by sitting on them so that they touch a warm, bare spot of skin on the parent (brood patch). Parental care of hatchlings tends to be intensive, with one or both parents feeding the young. Hatchling birds have a lot to learn (the parents' song, the location of feeding areas, migration routes).