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.
The breeding season is now almost over for Barn Owls, bar a few very late broods that are about to fledge. Following my early-season report, here’s an update on how birds fared at my study sites and those of BOCN volunteers in 2015.
Barn Owl productivity down overall in 2015
In 2014, Barn Owls were present at about 80% of traditionally-used nest sites monitored by BOCN and BTO volunteers across Britain. 2015 has also seen a very high level of occupancy, which is not surprising given last season’s bumper recruitment of juveniles and the mild winter that followed. But this year compares less favourably with 2014 when I look at chick ringing totals across my own regularly-monitored sites—a good proxy for breeding success. Last year, almost all Barn Owl pairs encountered fledged good-sized broods, and many had second clutches. I ringed 575 chicks that season, but this year that total is down to 110, similar to 2013, the four-fifths reduction almost certainly being caused by a predicted and observed decline in vole numbers.
(While a cycle of peak vole abundance normally occurs with a frequency of 3-4 years, I’ve always been uncertain about whether accompanying troughs tend to occur immediately after peak years or two years afterwards. The dramatic crash in vole numbers this year after 2014’s peak has further convinced me that the former is more common—the 2007 peak and 2008 crash being another dramatic example.)
This year, in Britain as a whole, I expect we will have ringed about 4,000 Barn Owl chicks: two-thirds less than 2014. This would be a little higher than the number ringed in 2013 (3,032), considered one of the worst Barn Owl breeding seasons since 1958.
But with big regional differences
Moving on from the overall picture, an intriguing aspect of the 2015 season is that unlike previous years there appeared to be very large regional differences in numbers of pairs of Barn Owls attempting to breed, characterised by an apparent gradient east to west across England. In Lincolnshire and East Yorkshire, for example, only 10-15% of pairs at traditionally-used nest sites attempted to breed and there was an 80-90% drop in the number of chicks ringed compared to 2014. Progressing west into Nottinghamshire, Warwickshire and Northamptonshire, and south into Norfolk and Cambridgeshire, 50% of pairs bred, though small brood sizes meant that ringed chicks were still only a quarter of numbers in 2014. Interestingly, birds were more productive along some estuaries and coastal river catchments in these counties: perhaps this was because declining vole populations backed up against these watersheds in early spring, providing better pickings for the owls there. In south Scotland and the English counties of Cheshire, Cumbria, Suffolk, Kent, Sussex, Buckinghamshire, Wiltshire and neighbouring counties, numbers of chicks ringed were at 65% of 2014 levels and, finally, in central Wales, the number of breeding pairs present and chicks ringed was actually higher than in 2014 (almost 120%), and the average brood size of 2.7 was significantly higher than elsewhere in Britain.
Other observations: older females absent
As many of you know, for the past 25 years my own main areas of Barn Owl study have been in the eastern half of England: east and mid-Yorkshire, Nottinghamshire, Lincolnshire, Norfolk, Cambridgeshire, Hertfordshire, Kent and Sussex. This year I found evidence of Barn Owls at 238 nest sites, 171 of which actually had adults present at the time of monitoring. I was able to sex and age 137 of these adults and while about a third of males were first-year birds (37%), this ratio went up to almost two-thirds (62%) for females. Why were so many older females apparently absent this year? My ringing returns suggest that Barn Owl mortality was relatively low during the mild 2014/15 winter, so it seems unlikely that they all died. Given how many underweight birds were found this year (see below), perhaps older females skipped breeding in greater numbers than younger ones, making little attempt to re-settle at their traditional nest sites and freeing them up for first-years.
Many females underweight
Of the 171 nest sites where I found recent evidence of Barn Owls (fresh pellets/feathers), I observed breeding at 83 of them, and 68 pairs of birds successfully produced one or more young, the average number fledged being 2.3. Most of the breeding females I weighed in April to June were 380-430g, while all 47 non-breeding females were below 360g (which tends to be the lower threshold weight for breeding) and most were less than 340g. Females were also at or close to 360g in most of the nests where breeding was unsuccessful: in many cases I found females present with abandoned clutches of 1-3 eggs buried beneath nest debris. Probably these females had had to leave their nests more often than usual to feed because their mates weren't bringing in enough, resulting in too little incubation.
Timing of breeding typical
I was able to determine first egg date for 70 nests via my established method of back-calculating from the age of the oldest surviving chick, and I found that while a handful of pairs had laid during the first two weeks of April, two-thirds had begun in a window from 20 April to 10 May. Apparently, a small surge in egg-laying then occurred between late May and mid-June, a time normally associated with repeat clutches, and a few pairs laid in July and August. This is when second clutches are normally laid, though this year these were probably late first attempts or repeats.
What might we expect in 2016?
In 2012 I estimated that the Barn Owl population in Britain had increased over the past 30 years from about 4,500 pairs to as many as 9,000, an average annual increase of about 5%, thanks to sustained conservations efforts. Although 2015 appears to have been a less productive year in most areas, if the high nest site occupancy recorded over the past two seasons carries on into 2016, better breeding conditions might make it another year of good recruitment. However, although vole numbers are on the rise again in some areas and should continue to rise through 2016, occupancy and breeding condition of owls could still be affected by a cold winter, which some forecasters are predicting.
During September and October, I visited several late nests in East Anglia and the Midlands and came across adult females and young weighing over 440g—very good weights—suggesting vole numbers are rising again in these areas. The vole cycle in parts of western and south-western Britain might be lagging by comparison, however, meaning that those areas may experience a trough in vole numbers early next year, affecting females' ability to get into breeding condition.
Overall, I doubt average brood sizes in most areas of Britain will be much larger than 2.7. It will be particularly interesting to see whether the first-year birds that took up many of the traditional nest sites this year will be ousted in 2016 by older birds that are ready for breeding once more.