Barn Owls in 2018: update from Colin Shawyer

26 Apr 2018

This year I thought it would be interesting to relay my findings on a regional basis. Most of you know that since the early 1990s I have been monitoring Barn Owl nests from Yorkshire through the eastern, midland and central southern counties of England to Kent and Sussex. The data gathered each year is collected from a core sample of the 3,500 nest sites I have installed over the last 25 years at which Barn Owl breeding occupancy usually ranges between 50 and 75% except in those years of low vole abundance. The figures that follow are derived from the five English regions of Yorkshire and Humberside, East Midlands, East England, Central South and South East England.

Understanding the dates at which the first egg is laid in any one year can provide a valuable indicator of the stage of the vole cycle, and when it occurs especially early, between March and mid-April, this can alert us to the prospect of higher than average clutch sizes and double broods later in the year.

Since analysing first-egg dates in the mid-1990s and later during the BTO’s 10-year Barn Owl Monitoring Programme (BOMP), I have been fascinated by the synchronicity that is seen in first-egg dates from counties as far apart as Yorkshire in the north to Sussex in the south.

In 2017, for example, although some pairs began breeding in mid- to late-March, the average first egg date across the five regions of England that I monitor varied between 13 and 15 April, a remarkably high level of synchronicity. This was 14 days earlier than the 20-year average first egg date of 29 April and was accompanied by suspended wing moult in many incubating females, giving rise to the possibility of them having second broods later in the year. But in the end, many did not double-brood and instead moulted at their nests in June, suggesting that they or their partners had failed to maintain sufficient condition.

Breeding occupancy in 2017 at traditionally-used sites (breeding at least five times in the past 20 years) also varied little—between 55 and 57%—in the three regions of Yorkshire and Humberside, East England and Central South England. But in the East Midlands and South East England, breeding occupancy was significantly higher at 70% and 75% respectively.  Brood sizes recorded at or close to fledging varied little in four of the English regions, averaging between 2.4 and 2.6, but was significantly higher in south east England, at 3.1.

The early breeding of many pairs in 2017 and some higher than average clutch sizes did not culminate in high fledging success—only pairs in south-east England came close to the 20-year average. The probable reason for this was very cold spring nights in late April and heavy frosts in the first week of May inhibiting males providing food to incubating females, in turn causing females to leave their nests in search of food and resulting in eggs chilling and failing to hatch. This hypothesis is supported by the fact that among pairs that laid later in June—mostly those that failed on first clutches/broods—fledging success was somewhat higher, averaging between three and four.

In Montgomeryshire, Wales, my colleagues recorded a particularly early breeding season for some pairs in mid-March, with average fledging success of four for first attempts and numerous broods of six, seven and eight in the south west of the county. For those pairs that laid eggs in March and early April and successfully fledged young, over 15% went on to produce second broods of five and six, although brood depletion inevitably occurred at these nests during the later months of the year.

Colleagues in Scotland also reported an early breeding season, with an average brood size of 3.8 for successful nests with almost 20% of the population double-brooding, a few pairs producing nine to ten fledged young over the two broods.

What to expect in 2018

I don’t remember receiving so many reports in early February of Barn Owls being paired up and demonstrating their preparedness to breed—I believe this may lead to high occupancy rates and higher than average fledging success in 2018. Moreover, judging by regional reports in England and Wales, vole numbers seem to have held up well since 2017, which I’d considered to be the peak of the latest 3-4 year cycle. It is also likely, however, that the cold and snowy weather experienced in many counties in late February may delay—like last year—what might have been an early start to the breeding season. Also, at altitudes above 200 m where February snow cover is more likely to have lasted 3-5 days, adult mortality may have been higher and surviving birds may take longer to get into breeding condition.

On the whole, I would expect first-egg dates this year to be a little later than the average of 28 April although birds in some southern counties may be earlier.

Colin Shawyer
Barn Owl Conservation Network founder and co-ordinator, UK and Ireland

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