All owls lay white eggs which suggests that they all evolved from a hole nesting ancestor. Elaborate markings to conceal the eggs from predators are not needed in dark holes, instead they are white and easier for the parents to see. The number of eggs an owl lays varies from species to species, year to year, and on the individual birds themselves. In general, larger birds have been known to lay fewer eggs, and birds from tropical regions have been known to lay fewer eggs than birds found in more extreme latitudes. In some cases, certain species of owls such as the barn owl or snowy owl have been known to increase the size of the clutch as preying availability increases, while other times they have abandoned breeding completely in years where their prey’s population has decreased.
Owl eggs are a spherical shape and in most species the female owl begins incubating as soon as the first egg is laid. The owl’s eggs are laid at intervals of at least a day, often more, resulting in what is referred to as asyncronised hatching, where the eldest owlet can be up to two weeks older than the youngest this way each owlet reaches its peak food demand period at different times, spreading the demand for food out over time. In lean years, the older and stronger owlets will survive, and the younger owlets that die off may even be utilized as a food source for the remaining owlets. Owls have been known to lay between one to thirteen eggs depending on the species and the season, however three to four eggs is usually more typical in most owls.
During the incubation process, and until the smallest owlet is large enough to maintain its own body temperature, food is provided by the male owl and is delivered up to as many as ten times a day. The female mother rarely leaves the nesting site and develops a sparsely feathered area on her belly known as a brood patch. This almost bare belly patch has a higher density of blood vessels than other parts of the skin and provides the eggs and/or young with direct warmth. Rather than providing food for her young, she dispatches the food and feeds the owlets small slivers of the food until they can swallow the prey whole, she then helps the male owl with hunting. Young owlets begin producing pellets as soon as they begin eating whole prey, or prey parts that consist of bones, fur, and other indigestible parts.
The age of fledging (learning to fly) varies greatly on the species, etc. and some owls have even been known to remain in one area until the following year. Eventually the young owlets learn to hunt and often start off by hunting and eating insects, and other food sources brought in by the parents, which sometimes may still be alive. Most species are independent by the time their first winter hits, and the parents in many cases have also been known to begin driving the owlets away prior to this.
For most owl species, breeding usually occurs during the spring. However, all the upbringing of the young and the period immediately following their fledging really depends on the weather, food availability, competition from other owls, disease, and availability of a suitable mate. Everything is usually timed to coincide with the maximum abundance of prey animals. Courtship rituals vary from species to species, but usually involve calling. In this case, the male owl usually will try and attract a female to a suitable nesting area by using special courtship flights, calls and offerings of food. Copulation follows the acceptance of food by the female. There is also often mutual preening where the pair perch close together.
In general, owls are monogamous creatures with pairs being compromised of one male owl and one female owl, neither of which is involved with any other nesting birds. Within some owl species, the pair bonds only for the duration of the breeding season. This especially depends on if the species is migratory. In other species such as the Little Owl, pairs may be known to remain together throughout the year. While, Tawny Owls have been known to pair up for life.
Owls are very territorial creatures. This often shows during breeding season. They will often be found vigorously defending their nests and surrounding the feeding territory against members of the same species, or against other birds who may try to compete for the same resources of food.
One way in which owls differ from other bird species is that they are not best builders. Rather, they prefer to use readymade nests by taking over other abandoned bird nests, or forcing a squirrel out of its nest. They are opportunistic creatures in this sense and will often even be seen breeding in open terrain on the ground. Holes in trees, hallows of tree trunks, holes in barns or other buildings, and/or natural rock crevices all make great nesting sites for many species of owls. As a general rule, owls will also usually try to reoccupy the same nesting territories in consecutive years.
Owlets hatch with the aid of an Egg Tooth (a unique protrusion on the beak, common to all birds, which drops off a week or two after hatching). Upon hatching, owlets are blind and have a thin coat of natal down. Within one to two weeks, a thicker second coat of down appears. This coat is known as mesoptile. As early as three to four weeks, some species of young may begin to leave the nest and clamber about. In tree nesting species, these owlets are referred to as Branchers. The next stage of development is fledging (mentioned above) this is where the owlets learn to fly. Fledging depends on the species. Some species begin to fledge as early as four to five weeks, while others take as long as nine to ten weeks to begin the process. Most owls reach sexual maturity and are ready to reproduce about a year after they hatch. However, in larger species breeding may not begin until their second or third year.