About the Barn Owl
Winter - hardship
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Winter has a natural culling effect on wildlife - and particularly affects species that are ill equipped to cope with periods of food shortage and sub-zero temperatures. Barn Owls originally evolved in a warmer and drier climate than we have in the UK so it is perhaps unsurprising that Barn Owl mortality peaks from December to March. During very extreme winters (such as 1947 and 1962/63) more than half the Barn Owl population of Britain was almost certainly wiped out and it would have taken several years for numbers to recover. In regions that suffer extremely cold winters every year, such as Eastern Europe, it is winter severity that largely determines the fluctuations in owl population from year to year. The main cause of death associated with winter mortality is, of course, starvation.
As late autumn and winter progress, the owls' prey becomes less abundant and the small mammals that survive become less active as temperatures drop. During periods of snow cover, voles and shrews stay underneath the snow, and although mice will move about on top of it, they do spend an increased proportion of their time under ground eating stored food. When snow cover is more than about seven centimetres deep and/or frozen hard the owls will have great difficulty finding and catching food. Many fail to find enough food and die. It's at times like these that some Barn Owls turn temporarily to unusual food sources such as small birds. During sub-zero temperatures, field voles are less active at night and more active during the day (when it's a little warmer). In order to catch them, and minimise loss of body heat, the owls too become more diurnal.
Compared to most birds that winter in Britain, Barn Owls are poorly insulated and need extra energy (food) during cold weather to make up for an increased loss of body heat. Increased winter rainfall is a problem too. Barn Owl feathers are very soft (an adaptation for silent flight) but not very water resistant so hunting during rainfall is avoided. Prolonged rainfall (which prevents hunting) alternating with periods of intense cold (which suppresses small mammal activity) can prove deadly.
The owl's highly detailed knowledge of its home range can make the difference between death and survival. During a short break in severe weather, a Barn Owl that knows its area really well can fly directly to the optimum hunting spot in any given ground, light, and wind conditions often making use of places where it can perch-hunt. This saves energy by reducing total flying time and heat loss (a perched bird loses less body heat than a flying bird). Local knowledge has so much survival value that the chances of a hard-pressed Barn Owl abandoning its home range to escape severe conditions are very small indeed. However, it may use parts of its home range that it doesn't visit much in the summer and may have favoured winter roost sites - perhaps because they afford more shelter or are close to a preferred winter foraging area.
Historically Barn Owls often inhabited or visited farm buildings that were infested with mice and rats allowing the birds to hunt indoors when conditions were severe. This tremendous advantage has been lost on virtually all farms due to changes in feed storage and rodent control.
The supplementary feeding of wild Barn Owls (using dead poultry chicks and/or mice) is possible, and can artificially increase the chances of survival but it is no substitute for providing prey-rich habitat.
Although low temperatures, snow cover, and rainfall can all have an impact on the owl's survival, they are usually much less important than prey abundance. Dr Iain Taylor's long-term study in SW Scotland demonstrated that Barn Owl survival was much more closely linked to vole numbers than it was to winter weather. Winters with moderate weather and low vole abundance produced more Barn Owl mortality than severe winters with high vole abundance.
There is more information about this topic in the Barn Owl Conservation Handbook.