About the Barn Owl
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There are numerous factors that determine how many Barn Owls there are in any given area. Some of them are fundamental things that apply generally to the whole population whilst others are locally variable factors such as a lack of nest/roost sites or perhaps the proximity of a major road causing very high local mortality. Because Barn Owls die in so many different ways and their food supply is affected by so many different factors, it is often difficult to attribute a lack of Barn Owls to any one factor and the relative importance of the various factors can be difficult to determine.
However, a range of research projects (including some carried out by the Barn Owl Trust) has helped to develop our understanding of this often-complex subject. Using a combination of science and common sense, and by allowing a degree of over-simplification, the following guide may be given:
The population level of any given land mass is determined by the relationship between productivity and mortality (the number of young produced and the death/survival of birds) and the import or export of birds. Landscapes where productivity exceeds mortality are known as "source" areas because some of the excess young disperse to other areas. In landscapes where mortality is greater than productivity ("sink" areas) the population level can only be maintained by birds coming in from source areas.
Some landscapes are more suitable for Barn Owls than others. Food availability, nest/roost site availability, and hazard density all vary tremendously both between and within landscapes. Food supply (which is largely controlled by the quality and amount of foraging habitat) is the main factor that determines both nesting success and survival. However, even in areas of optimum habitat, if there's a lack of roost/nest sites Barn Owls may be scarce or absent. In some landscapes (such as urban fringe) there may be plenty of good habitat and potential roost/nest sites but so many lethal hazards (major roads, overhead wires, etc.) that any Barn Owls that attempt to establish themselves are quickly killed.
Extreme weather can have a marked effect on sensitive species like the Barn Owl causing marked fluctuations in productivity and survival, and therefore population level. For example, low temperatures suppress small mammal activity which can make them much harder for auditory hunters to find leading to high mortality (e.g. March 2013). Additionally, Barn Owls are unable to hunt in all but the lightest rain so prolonged rainfall can be devastating (e.g. June 2012). The effects of weather on populations are generally short-lived as Barn Owls have a tremendous capacity to increase in number. However, population recovery can only occur if there are good breeding years (as well as poor ones) and mild winters (as well as severe ones). The increased frequency of extreme weather events from 2009 to 2013 had a devastating effect. More information
From egg to death, the Barn Owl life cycle can be separated into various stages and quantified (mean values) as follows: number of eggs laid (5.6), hatching success rate (4.5), nestling survival (3.6), fledging success (2.4), first-year survival (25%), and annual adult survival (63%). A major study by the BTO revealed that of all the various life-cycle parameters, first-year survival rate exerts by far the most powerful influence on overall population level.
There are of course numerous other factors that can influence population levels. Overall, food supply is considered to be by far the biggest factor and this is why the loss of good foraging habitat is thought to be the main reason behind the population's decline and habitat creation the most important conservation measure.
There is more information about this topic in the Barn Owl Conservation Handbook.