No owl can be said to build its own nest in the proper sense of the word. Burrowing Owls may extend an already existing burrow, and some of the large owls may scratch a slight depression or scrape in loose earth but nothing that compares with the marvels produced by some other birds. Nest sites vary but tree hollows and cavities are the preferred site for many species. The Elf Owl nests in holes in Sanguaro cactuses made by the Gila Woodpecker, the Burrowing Owl as mentioned in animal burrows, Grass owls trample tunnels and nest chambers in long grass, whilst larger species scratch a scrape or use old stick nests of crows, pigeons or diurnal birds of prey. The American Great Horned Owl often drives out quite large raptors such as red-tailed buzzards and takes over their nests. Tawny Owls often use old squirrel drays, and Barn Owls in Africa use the giant stick structures made by Hammerkops (a stork like bird) occasionally nesting communally. Many species can be encouraged to use nest boxes and they may even be used in preference over natural sites.
Many owls are quite territorial, especially during the breeding season, sometimes even other owls species are not tolerated, at the other extreme some species will defend the nest site its self but will share hunting habitat.
Breeding seasons are always geared so that rearing chicks coincides with the peak availability of food, this is not always when prey is most numerous but when it is able to be caught in increased numbers. This may be as a result of decreased vegetation cover, or when the prey are more active or vocal say in defence of their territories allowing them to be caught more easily. In some species this often means that the eggs are laid very early, some times with snow still on the ground, this also gives the relatively slow growing young more time to learn how to hunt more efficiently before the following winter.
All owls lay white eggs, which suggests that they all evolved from a hole nesting ancestor. Elaborate markings to conceal the eggs from predators aren't needed in dark holes indeed the whiteness may make them more visible to the parents. The egg above is being Candled. A light is shone through the egg so we can see the developing embryo. (The solid Pink Blob in the Middle) The blood vessels are forming to feed the developing chick and to allow Oxygen to be absorbed into the blood stream. The Yolk inside the egg isn't visible but is there to feed the baby as it grows.
The number of eggs laid varies from species to species, year to year and between individual birds. In general larger birds lay fewer eggs, and birds from tropical regions lay fewer than birds from more extreme latitudes. In temperate and sub-arctic regions some species like the Snowy Owl the Short-eared Owl and the Barn Owl can increase the size of the clutch as prey availability increases, in years when the lemming or vole populations crash, breeding may be abandoned totally.
Owl eggs are relatively spherical to a greater or lesser extent. In most species the female starts incubating as soon as the first egg is laid. The eggs are laid at intervals of at least a day, often more, resulting in what is known as asynchronised hatching, where the eldest chick can be up to 2 weeks older than the youngest. The means that each chick reaches its peak food demand period at different times and so spreading the load. In lean years the oldest and therefore strongest chicks at least will survive, the younger chicks, once dead, may even be utilised to feed the others.
During incubation, and until the smallest chick is large enough to maintain its own body temperature, all the food is provided by the male, the female rarely leaving the nest site. She dispatches the food and feeds the chicks small slivers until they can swallow the prey whole, she then helps the male with the hunting.
The age of fledging varies greatly and some species even remain in the area until the following year. Gradually the young learn to hunt, often starting on insects or food brought in by the parents, which may still be alive. Most species are independent by their first winter and in many cases the young are actively driven away by their parents prior to this.
A European Eagle Owl which has just hatched
A 3 week old European Eagle Owl chick with an Adult Bird in the Back ground.