About the Barn Owl
Spring - nesting
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Barn Owls can breed in their first year and the breeding cycle often starts in late winter. By early spring pairs are usually spending much of their time in and around their intended nest site. During courtship, or pair-bond reinforcement, the male spends more time hunting and presents additional food items to his mate or leaves them in the nest cavity. The female spends progressively less time hunting and as her weight increases from around 350g to 425g she comes into what's called "breeding condition". Copulation generally occurs each time food is presented and this, combined with her tip-top condition, helps to ensure all the eggs are fertile. However, it's not just a matter of food and sex. Most pairs also engage in mutual preening and cheek rubbing. They "talk" to each other in the nest making a wide variety of soft chittering and hissing calls, and their aerial antics (chasing while screeching) around the nest site can be quite energetic! Also, there's much individual variation. Nest cameras have revealed that some males are very active and attentive, some surprisingly lethargic, and others are hardly ever present. In almost every case the male is present at the nest site (with the female) in the two weeks leading up to egg laying and until the clutch is complete. This is when males are most vocal, defending their mate against other males and leaving only to hunt.
Although nesting has been recorded in every month of the year, most pairs lay eggs only in the spring. A study published in 1990 showed that the average date of the first egg laid was May 9th, however there is much annual and regional variation. Recent studies suggest that the first eggs are generally laid in early April - almost certainly a response to climate change. Eggs in March are now quite frequent so the main breeding season is normally quoted as "March to August" (inclusive). Early laying females are generally those with the best food supply. However, in the late winter/early spring period, small mammal numbers are at their lowest so prey availability is probably linked to improving weather conditions and increasing prey activity, which together result in increased prey "catchability". First-year birds tend to breed later than older more experienced birds.
Barn Owls do not "build" a nest but just before egg laying the female usually makes a shallow scrape in the previous years' nest debris and/or breaks up a few recent pellets creating a soft layer for egg laying. Where there is no nest "cavity" as such, (for example, on a wall top or loft floor) pellets may be widely scattered and the eggs laid directly onto any surface (stone, wood, loft insulation etc.). Compared to other owl species, Barn Owls lay small eggs (in relation to their body size) and they lay lots of them.
Most bird species don't start to incubate (warm) their eggs until the clutch is complete so the eggs hatch at more or less the same time. Barn Owls begin incubation as soon as the first egg is laid and additional eggs are added every two to three days.
The average number of eggs laid is 5.6 and clutches laid earlier in the year are usually larger than later ones. In cases where the eggs are abandoned this is almost always because the female is underweight and/or suffering extreme food shortage. Normally, after 31-32 days incubation, the eggs hatch in the order they were laid (at 2-3 day intervals). This is termed "asynchronous" hatching. The average hatching success rate is 4.5 and the age difference between the oldest and youngest nestlings can be as much as three weeks.
The female normally does all the incubation and once the eggs have hatched she continues to sit, brooding the young until the eldest is around three weeks old. By this time she has been sitting almost continuously on the nest for about nine weeks and may be quite heavily soiled. At this stage it is not uncommon for breeding females to be found drowned in cattle troughs where they have attempted to bathe unaware of the dangers of deep steep-sided water containers which can be death traps.
What affects the success?
By late spring, small mammals have completed their first breeding cycle (thus increasing the owls' food supply) and nestling Barn Owls are usually half-grown (around 5 weeks old) at which stage they are generally heavier than a fully grown adult. The average brood size is 3.6 but this figure can vary from zero to seven and occasionally even more. Clutch size and brood size are directly related to food supply - the more food the adults have the more young they produce. Food supply is affected by habitat quality, prey density, the weather and the experience and behaviour of the individual adult owls.
Although food supply is undoubtedly the biggest factor, nest safety can be a major issue at some sites. Some nest places are relatively unsafe and young Barn Owls that fall from the nest and are unable to climb (or fly) back up usually die. As well as advising on the creation and management of good habitat, the Barn Owl Trust has developed safer nest box designs that help reduce nestling mortality...
There is more information about this topic in the Barn Owl Conservation Handbook.