Most birds have a regular breeding season, timed to coincide with the most abundant season of the year, but some, particularly those adapted to unpredictable climates, are opportunistic, only breeding when conditions allow.
The largest species may breed every two years, but most attempt to breed at least annually, some raising several broods over the breeding season.
Birds build their nests of several substrates; the most important issues are protection from predators and the elements. Therefore, species that nest on predator-
The nest itself must hold and shelter the eggs; in form, they vary enormously from simple scrapes in the dirt to large complex stick nests and hanging structures, woven together or glued with cobwebs or mud, and lined with soft material. Tree holes and holes in banks or cliffs also make good nests but competition for them may be fierce.
The megapodes construct a mound in which they bury their eggs and maintain the temperature at about 90–95°F (32–35°C) by scratching soil on or off. Many species use the same nest area, nest site, or actual nest year after year.