Barn Owls in 2014 and a prediction for 2015
Widely recognised for his work on the study and conservation of owls and raptors, professional ecologist Colin Shawyer has collaborated with the BTO on projects such as Project Barn Owl (1995-1997) and the Barn Owl Monitoring Programme (2000-2009). As founder and co-ordinator of the Barn Owl Conservation Network (BOCN), Colin is in regular contact with Barn Owl experts across the country and oversees the annual monitoring of over 3,000 Barn Owl nest sites.
Since publication of the Barn Owl Survey of Britain and Ireland in 1987 and the founding of the Barn Owl Conservation Network, BOCN Advisers, many of whom have also become BTO ringers since that time, have made enormous strides for the conservation and research of this bird. Not only have they and others seen their local Barn Owl populations increase but have contributed importantly to Project Barn Owl in the mid 1990’s and more recently to the BTO’s Barn Owl Monitoring Programme (BOMP).
How many sites are we now covering?
By the end of 2014, BOCN advisers together with their BTO ringing colleagues had monitored in excess of 4,000 active breeding sites in Britain with chicks having been ringed at just over 3000 of these. Together with my colleagues Peter Wilkinson, Paddy Jackson, Bob Sheppard and Alan Ball, all of us now well into our 25th year of Barn Owl monitoring, 2,400 Barn Owls were ringed by us in just three counties of England. My own work also takes me to other counties, such as Yorkshire, Nottinghamshire, Norfolk, Hertfordshire, Sussex and Kent where an additional 300 Barn Owls were ringed. Collectively, therefore, we monitored 700 active breeding sites in 2014 and an additional 2,000 which either had roosting birds or were inactive. Having installed over 3,000 artificial nest sites I myself have needed to adopt an annual sampling regime, similar to that I started in 2000, the first year of BOMP.
Vole numbers and site occupancy presage bumper 2014 breeding season
I had predicted that in 2014 field voles would be at their peak of abundance and by the late summer of 2013 we were already experiencing a rapid rise in the numbers of this small mammal. Although this upturn had come too late for Barn Owls that year it boded well for 2014 and we were not disappointed. By February 2014 reports of vole plagues, the like of which were commonly seen in the late 1800s and early 1900s, were being reported from counties as far apart as Lincolnshire and Sussex and as a consequence Barn Owls were seen performing their daytime courtship rituals particularly early, in late January.
Although we found occupancy levels at traditionally-used breeding sites were down by about 20% in 2014, a likely reflection of the unprecedented winter mortality the year before and the poor breeding year that followed, this was almost entirely compensated for last year by first year birds occupying new and previously unused nest sites. Some of these sites had not been frequented since they were first installed many years ago. Interestingly, of the 98 breeding adults I caught in 2014, 33% were first year birds, the rest being adults aged between 2 and 13 years.
Early laying dates, high brood sizes and double brooding
Average first brood sizes in northern, eastern, midland, central and south eastern counties of England, ranged between 3.6 and 4.0 per successful nest. In those areas where dedicated monitoring for second broods was carried out and where this could, therefore, be calculated with some certainty, we found 45% of those pairs which produced first broods were successful, second time around. For these birds clutch sizes were, as usual, larger than firsts with some birds having laid clutches of 10-13. In spite of this, the average second brood size at the time of ringing was 3.8, brood depletion for second broods almost always being greater than firsts, males at this time being in wing moult and generally less able to make sufficient prey deliveries to their young. At a few nests we found that partners had changed between the first and second breeding attempts but because this relies on identifying the adults on both occasions the extent of this behaviour in Barn Owls is as yet unknown.
Barn Owl with eleven eggs by Colin Shawyer.
Whilst brood size is not strictly synonymous with that of fledging success, brood depletion after ringing is normally low provided climatic conditions remain stable during the summer months as they did in 2014. Not surprisingly, brood size and the proportion of second broods produced, are the highest we have recorded since our monitoring work began in earnest 20 years ago. Also, although a significant number of Barn Owls in Britain began laying in March, the average first egg date for Barn Owls in 2014 was 4th April, 24 days earlier than the 20-year average and the earliest we have experienced.
Although the ringing data for the UK will not have been collated yet, the prediction I made in late 2013 for a bumper year in 2014 and the expectation that over 10,000 Barn Owl young would be ringed will, I think, prove true and may well exceed 12,000, the highest ever recorded. Assuming we are ringing on average, about one-third of all Barn Owl chicks in the UK which is indicated by recent research that I have conducted, then we could consider that about 30,000 young were produced in 2014.
Contrast to 2013
As we were all too aware, the situation in 2013 was quite different. The field vole was in its trough of abundance and the shortage of food together with the extreme weather conditions during the autumn of this year and in the following winter in February and March, conspired to depress breeding success to the low levels perhaps last seen in 1958. Nest occupancy in Britain was particularly low with many pairs which survived the harsh winter choosing not to return to their traditional nest sites from their usual winter roosts and weighing in at 50g below normal.
For those pairs that did return to their nest sites in 2013, breeding did not begin the following year until June and July. Many of these were first-year birds which fledged late in 2012 following the high mortality of nestlings due to unprecedented levels of rainfall earlier that year. When the data were eventually analysed, 80% of traditionally-occupied nest sites had remained vacant in 2013. The average brood size in those pairs which finally managed to breed was between 2.6 and 2.9, this figure being buoyed up by the late broods some of which eventually managed to produce 4-7 young. This followed a surge in vole numbers which appeared to begin in July of that year but came too late for most breeding pairs of owls.
Timing of the first egg date in Barn Owls is the best early indicator of how the breeding season will progress. Early laying, like that in 2014, normally results in large sized clutches, good sized broods and for a good proportion of pairs the opportunity of double brooding, all of these outcomes being primarily dependent on the phase of the 3-4 year cycle of field vole abundance and in particular the abundance of this small mammal just prior to breeding.
Providing we do not experience prolonged periods of snowfall and extreme cold during March I think we can anticipate that in 2015 high numbers of young birds will be recruited into the breeding population. In this respect it will be particularly interesting to see if those produced from second broods last year will be found breeding as early as April and May this year. I imagine egg laying will begin for most pairs at much the usual time towards the end of April and in early May and although vole numbers will undoubtedly be declining from their 3-4 year peak in 2014, site occupancy should be high and return to its peak level. Brood size, on the other hand is likely to decrease significantly this year to about 3.0 and in 2016/17 will fall further as the vole numbers reach their trough.
Nest monitoring made easy
Our nest monitoring work in Britain today is assisted by the establishment of a large number of Species Recovery Areas (SRA) out of which the now extensive habitat matrix of Barn Owl Recovery Networks (BORN) has been born and throughout which nestboxes have been installed at regular intervals. As I have previously reported we currently estimate that 75-80% of the UK population now breed in nestboxes. As well as contributing to the conservation success story, artificial nest sites provide an invaluable facility to enable the safe handling of chicks and adults and a means by which this bird can be monitored routinely and with minimal disturbance. A number of our BORNs also form part of notable study areas where detailed nest monitoring is yielding greater insights into the biology of this bird. Like national surveys undertaken in the past, this enables us to fine tune our UK conservation strategy, drawn up and adopted in 1988 by the BOCN and by the RSPB and JNCC in 1992.
Status of the UK breeding population
The cyclical changes in Barn Owl nest occupancy and breeding productivity we have been seeing are of great interest but have little effect on the British breeding population, which I believe has remained relatively static during the last few years and which, in 2011, I conservatively estimated at about 9000 pairs. This is at least double that reported by me in 1987 and by the BTO in 1997 and is a very reassuring trend.
In some areas of eastern England today, especially where concerted conservation action has been taken to enhance rough-grassland, provide connectivity of this habitat, and to install new and replace old nestboxes, Barn Owl numbers will be sustained. Elsewhere, providing we continue to understand and take appropriate conservation and research actions to address those factors that are limiting population growth in this species, there is every reason to suppose that Barn Owl numbers will continue to increase over the next decade or so.
24 February 2015
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