About the Barn Owl
Autumn - dispersal of young
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At eight weeks old a typical fledgling Barn Owl has already learned to pounce but struggles to fly, at nine weeks it's flying fairly well, and by ten weeks it starts perfecting the art of landing, accurate pouncing, flying and hovering in various wind conditions. By eleven weeks many have made their first prey capture and some have already started to disperse. From 10 to 14 weeks the amount of food each one receives from the adults goes from an average of four small mammals per night down to zero. An owlet that continues to spend time around its parents (beyond c. 14 weeks old) may be chased away although lingering somewhere within the parental home range may be tolerated for longer as Barn Owls are not particularly territorial.
Where to go?
Considering the population as a whole, the direction in which individual juveniles disperse appears to be completely random. However, at the local level it is probable that dispersal direction is influenced by landscape (the presence of things like mountains, huge dense forests or open water). Radio tracking studies have shown that post-fledging dispersal usually consists of a series of moves between temporary roost sites (or clusters of closely-spaced sites) that are occupied for between 3 and 15 weeks and are up to 1km apart. Presumably the birds are trying out different areas and settle when they are content.
The factors that cause young owls to stay or move on are almost certainly food supply, the availability of dry roost sites (either undisturbed, or where they can stay out of sight), and perhaps the presence of an unpaired Barn Owl of the opposite sex. In Britain dispersal distance is not affected by annual fluctuations in food supply, but in mainland Europe Barn Owls disperse further in years when food is less abundant.
The dispersal period ends in late November. Birds that haven't established a permanent home range by this time are probably forced to do so: reduced prey availability and deteriorating weather making survival the highest priority. Survival is linked not only to food supply but to the species' highly sedentary behaviour. Barn Owls that stay in one place and get to know the landscape in great detail stand a better chance of survival than those which keep moving on. This is due, in part, to the sensory limitations of nocturnal foraging.
The ringing of nestling Barn Owls and the subsequent recovery of ringed birds has provided a wealth of information* on where they go, how long they live, and why they die. Although ring recovery data is largely based on birds that have died, it does provide an insight into what happened while they were alive. The average dispersal distance is 12km (7.5 miles). The majority move less than 10km and a small minority move a very long way indeed. Unusual movements include Devon to Wiltshire, Devon to Yorkshire, and Germany to Cornwall.
Barn Owls possess no homing instinct: dispersing young almost always move beyond their parents' home range and virtually never return. Sites that appear to have the same resident pair for many years are, in fact, only maintained by incoming juveniles - each time an adult dies it's being replaced by a juvenile dispersing-in from a different nest site.
People who create ideal habitat and erect a nesting box often wonder when their site will be occupied. If you don't already have Barn Owls around then the chances are that it will happen in August-November and the Barn Owl moving into your site will be a juvenile in post-fledging dispersal. At this time it is important to avoid undue disturbance because a bird that has only recently occupied a site is more likely to move on than a well-established older bird.
It is a sad fact that juvenile Barn Owls in dispersal are more likely to die than establish a home range. There are two major factors working against them. Firstly, their relative inexperience in finding and catching food means they are more likely to starve to death than adults. Secondly, the further a bird moves across the countryside the more man-made hazards it encounters. Collisions with traffic, flying into overhead wires, and drowning in cattle troughs are all too common.
The Brood of 101
The Barn Owl Trusts' biggest-ever research project (on the subject of Barn Owls and Major Roads) included a detailed investigation into dispersal behaviour and mortality. Within the 109-page report we produced a 2-page section called "The Brood of 101" which, if you are really interested in Barn Owl dispersal and mortality, is well worth reading. The real enthusiast may also like to read Chapter 8 of "Barn Owls and Major Roads: Results and Recommendations from a 15-year Research Project".
From a conservation point of view, juvenile dispersal mortality and survival is a very important subject because the survival rate of juveniles has more effect on the overall population level than any other life-cycle parameter. In other words, the numbers of eggs laid, hatching success, nest mortality, and adult survival rate, have less effect on the population size than juvenile survival.
* Please note: ring recovery data is biased towards birds that die in conspicuous places. Thus, causes of mortality such as starvation and poisoning are relatively under-recorded.
There is more information about this topic in the Barn Owl Conservation Handbook.