Barn Owls and Winter Snow

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Article by Colin Shawyer on > www.bocn.org

A dead Barn owl lies in the snow (Colin Shawyer)

Climate is the fundamental natural influence regulating a species’ distribution and abundance and acts largely by governing the availability of food and shelter. Bird communities will respond to the effects of both short and long-term changes in climate and can provide valuable empirical indicators of these changes which the BTO’s Barn Owl Monitoring Project (BOMP) is designed to detect.

For the Barn Owl, which in Britain is at the northern limit of its world range, changes in climate could be expected to exert noticeable effects on adult survival rates and breeding productivity. Should these climatic events be sustained over a prolonged period, changes in the Barn Owl’s population size and distribution in Britain and Ireland, are a likely consequence.

Snow cover and rainfall can, for different reasons, reduce the Barn Owl’s efficiency to capture prey. It might be anticipated therefore, that during mild winters adult survival and breeding productivity would both be high and should these climatic events occur over a prolonged period of years that the population would increase and that those parts of Britain previously considered marginal for Barn Owls, would be colonised. Conversely severe winters might result in lower survival rates and reduced breeding success and lead to a decline in the Barn Owl population, particularly from those areas of higher altitude and more northerly latitude.

During the Barn Owl Survey of Britain and Ireland I sought to test this hypothesis. I analysed the BTO’s ringing data to provide a measure of annual breeding success alongside the snow statistics of the Meteorological Office.

The ringing data between the years 1914 and 1985 demonstrated annual peaks and troughs in Barn Owl numbers with a regular periodicity of 3.4 years and because it was largely nestlings which were ringed at this time, provided a measure of annual breeding productivity.

The periodicity of about three years which was evidenced in the ringing data, is largely a result of the cyclical changes of abundance which naturally occur in the field vole population which in most years appears synchronous throughout large areas of mainland Britain.

Whilst the vole cycle largely governs the annual breeding success observed in Barn Owls, climatic extremes, particularly those which occur during winter (snow lying) and early spring (prolonged rainfall) can also have a part to play by depressing the Barn Owl’s breeding productivity.

The number of days during any one winter when snow blanketed the ground (days snow lying) was gathered from low-lying met stations in Britain over a similar period to the ringing data. This showed that in those years when the number of days snow lying was greatest, breeding productivity was at its lowest.

This encouraged me to look at how winter weather patterns might have changed in the 20th century and how this might have influenced the long-term decline in Barn Owl numbers which were being reported during that century. Winter severity, as expressed by the number of days snow lying in any one winter, showed some unexpected trends.

Although snow statistics were not gathered prior to the 20th Century, a combination of the met office’s winter temperature and precipitation data clearly demonstrated that the 40-year period, 1860-1900, was one of extreme winter severity.

In complete contrast, winters during the 38-year period from 1901 to 1938 were remarkably mild with only one of these exceeding 20 days snow lying. Between 1940 and 1986 winters became severe once more with almost half of the 47 winters exceeding 20 days snow-lying. This was partially reflected not only by a 70% decline in the Barn Owl population during this period but probably why 82% of the 3000 breeding sites notified to the survey were recorded at altitudes below 100 m, a higher proportion to that reported by the 1932 Barn Owl census.

After 1986 and until 2008, winters once again became remarkably mild with none of these exceeding 20 days of snow-lying. This is likely to be one reason why a greater proportion of Barn Owls are now reported breeding at higher altitudes and at more northerly latitudes than they were during the 1982-1985 survey.
In 2009 and 2010, in spite of global warming, we are experiencing snowy winters again. History has suggested that since the mid 19th century they have come in blocks averaging about 30 years, so could we be facing another prolonged term of hard winters once more?

What then are the implications of this particular winter on Barn Owls? For the dark-breasted race of Barn Owls T.a.guttata in our neighbouring countries of Belgium, Holland, and elsewhere in north-western and central Europe where the mean January isotherm is below 4oC, these populations may cope quite well since although this race faces similar difficulties to our own of locating its prey beneath snow, it is considered better adapted to the cold.

For our white-breasted race in Britain and Ireland, however, because 20 days snow lying has already been exceeded in 2009/2010 we could anticipate some reduction in nest site occupancy this year as a consequence of higher than average adult winter mortality. Indeed by the eighth day of this snow event, I began receiving reports of dead Barn Owls from various regions of Britain. However, because the vole population began increasing in 2009 following its trough in early spring and is likely to peak during 2010, those Barn Owls which do survive the winter would be expected to lay eggs earlier than usual and produce larger than average broods. Overall therefore, the impact of this severe winter on Barn Owl numbers in Britain may be nothing like as high as it would have been if we were in a low phase of the vole cycle.

Today because the Barn Owl population in Britain is both more abundant and more widely distributed than it has been since the 1980s it is likely to be far more resilient and able to withstand the onslaught that harsh winters can bring. The establishment of a viable and sustainable Barn Owl population was after all the primary aim of the research and conservation work which culminated in 1988 in the formation of the Barn Owl Conservation Network (BOCN) which so many of you have successfully contributed to over the last 22 years.

I am currently responding to numerous enquiries about the supplementary feeding of wild Barn Owls and advising on the pros and cons of this as well as directing the public where to send carcasses and details of ringed owls. Please email me with your observations and experiences. It all helps to build up a picture about the impact of these climatic events, the like of which we have not seen for over two decades.

For those who are interested in this topical subject, the results of the Barn Owl Survey I conducted between 1982-1985 and which was published in 1987 as The Barn Owl in the British Isles, its Past Present and Future discusses in much greater detail how climate and climatic extremes such as snow, rain and drought, can effect Barn Owl numbers in Britain and Ireland.

Colin Shawyer
BOCN Coordinator Britain and Ireland
BOMP Project Development (Wildlife Conservation Partnership)

01582 832182
07774 899344
colinshawyer @ aol.com

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