About the Barn Owl
Roosting and nesting places
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Types of sites...
The question is often asked, "what did Barn Owls use before there were barns?". The answer must be tree hollows & rock crevices and these types of site are still used today. Farming in Britain started around 3,500 BC and as soon as haystacks, animal shelters, and houses were created Barn Owls almost certainly moved in. Indeed, the remains of Barn Owls have been discovered during the excavation of several human settlements including the iron age village of Glastonbury (Somerset) and a roman site at Woodcutts, Cranbourne Chase, Dorset.
In Britain the vast majority of occupied sites are agricultural buildings, particularly old stone, cob, or brick-built barns and stock sheds. However, due to changes in farming practices the vast majority of these have become disused and most of the traditional farm buildings that existed in the 1800's have already disappeared. Many of those that still exist are in terminal decay or undergoing conversion to some other use. Fortunately, Barn Owls have absolutely no sense of architecture and may use any rural structure that provides shelter and a suitable perching place. Although the majority of recorded nest sites are now in purpose-made nestboxes, Barn Owls still use a very wide variety of site-types and there is enormous variation in their suitability. Not every old barn is a potential nest site and even some of the purpose-made nestboxes are far from ideal!
The proximity of a good foraging area is obviously important but in terms of the sites themselves, there are some basic criteria that appear to be important:
Whilst roosting, Barn Owls like to either be hidden from view or undisturbed. Provided that the owl can hide, it can easily become accustomed to almost any type of regular human activity or noise. They seem to prefer perching on wood rather than metal or stone (presumably for comfort and minimum heat loss) and well-used roost places are almost always where the owl can stay completely dry during wet weather. Given a choice of perching places a Barn Owl almost always perches as high as possible - normally at least 3 metres (10') above ground level.
Barn Owls do not build a nest. Therefore (for egg-laying and incubation), they need a cavity-bottom or ledge that's fairly level. For example, in an old barn the nest may be on a wall top or between bales, in a derelict house it may be in an old (dry) water tank or in the loft. To use any site, the owl has to be able to get in but the absolute minimum hole size required is only 70x70mm. The minimum width of a nest ledge is around 250mm and the minimum floor area of a nest cavity is about 0.1sq. metres (300x300mm). If you are thinking of creating a suitable access hole or space for nesting please don't use these minimum figures as a guide!
As previously mentioned, not every old barn is suitable for Barn Owls. During the Trust's Barn Conversion Research Project (carried out in SW England), the number, type, suitability, and rate of loss of potential Barn Owl sites was quantified. Only 48% of traditional farm buildings were suitable for nesting although 76% of them were suitable as roosting sites. Amongst modern farm buildings, 41% were suitable for roosting in and only 4% had a potential nest-place (unless a nestbox was provided). Only 2% of all potential nest places were tree hollows.
In the UK, the types of sites used by Barn Owls for nesting show a clear east-west difference. In the western half of Britain the vast majority of recorded nest sites are buildings whereas in some eastern areas about 70% of sites are tree hollows. A well-known Barn Owl author stated that Barn Owls in the west selected buildings rather than trees because of the higher rainfall in western areas and extra shelter that buildings provided. A study by the Barn Owl Trust suggested that there was no positive selection - that the type of sites used by the birds was simply a reflection of types of sites available to them. Scientific research by Dr Iain Taylor later supported this view.
The Barn Owl Trust also investigated whether or not Barn Owls positively selected isolated sites (away from human habitation) or avoided sites in busy farmyards or close to human habitation. Again, the findings suggested that there is no positive selection. The types of sites occupied by Barn Owls are simply a reflection of what is available. Provided that the birds can remain out of sight, they seem perfectly content in sites with lots of human/agricultural activity. Some farmhouses have had Barn Owls nesting in the attic for many years.
Once settled into their home range (after post-fledging dispersal) Barn Owls generally use the same nest and roost sites for the rest of their lives. In areas where there are plenty of suitable-looking sites for the owls to choose from, it is often unclear why they choose the sites they use and ignore others. Interestingly, when a home range is vacated for a period (because the owls have died) and later re-occupied by new owls, the sites they choose to use are often the same ones the previous residents used. This suggests that occupied sites not only have a certain appeal to individual owls, they also have "universal" appeal to Barn Owls as a species. This fascinating aspect of Barn Owl site-selection has yet to be investigated more fully.
These days, you are more likely to come across Barn Owls roosting or nesting in a purpose-made nestbox rather than a natural or semi-natural situation. However, the occupation of sites like tree hollows, rock crevices, and dark corners in derelict buildings can easily go undetected so if you are involved in any type of development that may affect such places please take advice...
There is more information about this topic in the Barn Owl Conservation Handbook.