The period within which the species lays and incubates its eggs and rears its young to the flying stage. Sometimes also extended to include the period of mating and nest-building. Most species of birds lay eggs only during a few weeks or months of the year in any particular area. This timing of the breeding season is dictated by factors such as climate, availability of breeding sites, food availability and the physiological condition of the bird. For example in the western Europe almost all factors (of which food is often the most important) combine, ensuring that nearly all species breed in spring and early summer. Even in the tropics where large areas show little environmental changes throughout the year, seasonal breeding is the rule.