SCBOP Report for 2014

Suffolk Community Barn Owl Project – Report for 2014

Steve Piotrowski and Alec Hillier

Prologue

This is an update to the report on the project originally published in the Harrier by Hillier (2012). It describes the background to the project, the changes to the environment for the Barn Owl and brings the results up to date to the end of the 2014 breeding season.

Introduction and Background

The Problem

The Barn Owl (Tyto alba) has long been a familiar and much-loved Suffolk bird and country folk, especially those living in more rural areas, often still refer to it as the “white owl” or “screech owl”.  Suffolk’s breeding population was estimated to be around 345 pairs in 1932, but by 1985 its numbers had dropped by 57% (Shawyer 1998).  This dramatic dip in the population was largely due to the loss of grassland, which resulted in a corresponding loss of feeding opportunities.  Nesting sites too were becoming few and far between as old mature trees were removed as a result of Dutch Elm disease and, in some cases, as a preventative measure to prevent crops being shaded.  And many old farm buildings, which were formerly used for nest sites, were demolished or converted to houses and offices.   A survey of raptors and owls completed from 1995-1998 revealed that Barn Owl population had declined further to around 51-95 pairs (Wright 2001).

The Two-part Solution

1 Agricultural changes aid partial recovery

To sustain good Barn Owl numbers there must be enough prey, year round.  Field or Short-tailed Voles (Microtus agrestis) need a particular type of habitat – rough, tussocky grass that they can move through in tunnels and that provides their own source of food and nesting habitat. This habitat, in close proximity to a suitable nesting site, provides the ideal conditions for the owls to breed successfully.

Set-aside was introduced by the EU in 1988, a scheme that was to become compulsory in 1992.  Its aim was to help reduce the large and costly surpluses produced in Europe under the guaranteed price system of the Common Agricultural Policy.  Although environmental benefits were not its principal objective, it did help with the recovery of the Barn Owl as feeding opportunities increased when new large areas of grassland (as set-aside) became available.  Their cause was further helped by the introduction of agri-environment schemes such as Environmentally Sensitive Areas (1987) and the Countryside Stewardship Scheme (1991).   Both schemes were superseded by Environmental Stewardship in 2005 and this year will change to a new scheme again under the heading of Countryside Stewardship!  There is provision within these schemes for payments to farmers who are willing to establish grassland habitats by way of grass margins at field edges, rough grassland in awkward field corners and scrub management.  Such habitats are ideal for small mammals; in particular Short-tailed Voles.

2 Nest site provision completes the recovery

With more feeding opportunities becoming available it was time to look at the other ingredient to help re-establish a sustainable breeding population. That was the provision of nesting opportunities. The project decided to install specially designed Barn Owl nest boxes.  Initially, our ambitions were quite modest as it was planned to install only 90 specially-designed nest boxes during the period 2006-2011.  However, the willingness and enthusiasm of landowners and of general public was vastly underestimated and, by the end of the project’s first year, around 250 boxes had been either fixed or inherited.  Boxes were built using sustainable timber and the work contracted to a local charity (Special Objectives for the Local Disabled – SOLD), providing work for disabled people and later to a local prison. The boxes were installed by professional tree surgeons.  By the start of this 2015 monitoring season the project was monitoring 1698 boxes, mostly  in Suffolk and a few just across the county borders in Norfolk and Essex.  Sites hosting good feeding opportunities for Barn Owls were chosen such as nature reserves, farmland and community spaces like village greens and school grounds.

By providing a connected network of good habitat and nest sites we can give them the fighting chance they need to thrive.

The Project

SCBOP is dedicated to the conservation of Barn Owls. Originally managed by SOG the project passed to SWT and has now been returned to SOG for management. The principal partners are Suffolk Wildlife Trust (SWT), Suffolk Ornithologists’ Group (SOG) and British Trust for Ornithology (BTO), but a number of smaller independent projects also fall under the SCBOP umbrella including those administered by Dedham AONB, Stour Valley Project, Thornham Owl Project and Suffolk Owl Sanctuary. The project provides advice on habitat enhancement and nest box siting to land owners who have suitable habitat and a monitoring service for the boxes. This system of raising awareness, creating nesting opportunities and managing suitable nearby habitat is having a positive effect on Barn Owl populations across Suffolk.

The project involves the whole community and the boxes are made by local organisations and monitored by an army of expert volunteers each year. The county has been divided into thirteen areas each with a coordinator. These coordinators manage the 120 monitors who hold permits entitling them to disturb the schedule 1 listed Barn Owls for the purposes of data gathering. A large proportion of the Barn Owls monitored are ringed by BTO trained and licenced ringers enabling the birds to be individually identified and their movements tracked.

This is a project of which all Suffolk people can be truly proud.

The effectiveness of the project is shown by the two maps displaying the distribution of all boxes Barn Owl occupied boxes in 2007 and 2014 in figures 1 and 2 respectively.

Figure 1 Barn Owl Box Locations and Activity 2007

Figure 1 Barn Owl Box Locations and Activity 2007

 

Figure 2 Barn Owl Locations and Activity 2014

Figure 2 Barn Owl Locations and Activity 2014

Monitoring results

The previous report in the Harrier showed that the number of boxes and the number of Barn Owls increased year on year showing a great success rate. The tabulation extends the information to 2014.

Number of Sites 2007 2008 2009 2010 2011 2012 2013 2014
Barn Owl Pellet 11 25 53 40 54 36 37 30
Barn Owl Adult 12 34 57 55 50 76 151 57
Barn Owl Egg 0 17 3 5 4 12 13 14
Barn Owl Egg Failed 9 19 23 13 16 19 10 9
Barn Owl Young 77 102 123 149 201 289 66 297
Barn Owl Young Failed 4 4 3 4 6 12 4 5
Barn Owl Total 113 201 262 266 331 444 281 412
Tawny Owl 8 13 21 25 22 21 11 22
Little Owl 1 16 18 16 11 12 8 15
Kestrel 12 23 24 25 17 20 14 21
Stock Dove 71 112 239 253 317 263 325 289
Jackdaw 63 83 141 177 189 206 176 266
Grey Squirrel 6 12 17 8 9 25 28 13
Empty 90 117 199 239 289 236 340 325
Not Checked 203 167 117 163 104 187 353 306
Other 4 0 11 9 7 4 9 4
Number of Active Sites 571 744 1049 1181 1296 1418 1545 1673

Table 1: Nest Site Occupancy for 2007-2014

The sudden drop in Barn Owl presence in 2013 is noticeable. Table 1 also shows that the nest sites provided by the boxes are used by other wildlife with Stock Dove (Columba oenas) an amber listed species being particularly assisted.

2013’s poor breeding season

As is so often the case, winter weather influences Barn Owl populations and was the main cause of the 2013 decline. The winter (as defined by the Met Office as December 2012 to February 2013) and the spring (March, April and May) of 2013 being 0.6 and 1.7 degrees colder than average. This put a great stress on the Barn Owls and on their preferred prey the Short-tailed Vole. Mortality was extremely high and reports of dead owls were reaching the BTO at up to 19 per day.  These figures involved only ringed birds that were reported, so we were seeing just the tip of the iceberg with a significant proportion of the UK’s population perishing. Many of the birds which did survive were poorly fed and only a few females were able to reach breeding condition.  The table below shows the weather averages for East Anglia for the winter and spring of 2012, 2013 and 2014.  Note the Act column shows the averages whilst the Anom is the difference from the average. Both 2012 and 2014 were warmer than the average whilst 2013 was colder.

Table 2 Weather Data for East Anglia sourced from Met Office Website

Table 2: Weather Data for East Anglia sourced from Met Office Website

2014 Bounce Back Year 

As seen above, the winter and spring of 2014 were warmer than average, so the Barn Owls were less stressed by the winter weather and then the warm spring yielded an early growth of grass which encouraged a swift recovery in the vole populations, thus providing a much-needed food.

This meant that Barn Owls could begin nesting earlier than in normal years.  Shawyer (1998) has pioneered the development of an ageing technique for nestlings which involve the measurements of the seventh primary feather recording the distance from feather tip to the point of emergence from the waxy feather sheath, or where the feather has not yet emerged the length of the pin.  These measurements can be compared with a table and the pulli’s age plus or minus one day can be calculated.

Hatch dates for the 7 years for which the project has sufficient data are shown in Figures 3 and 4 below.

Figure 3 Hatching Dates of Barn Owl Pulli Ringed

Figure 3 Hatching Dates of Barn Owl Pulli Ringed

Figure 4 Distribution of Hatching Dates of Pulli Ringed

Figure 4 Distribution of Hatching Dates of Pulli Ringed

Anecdotal evidence from monitors suggested that there were large stores of food in the boxes where Barn Owls were breeding and it appeared that the pulli were heavier. However plotting weight against age for the pulli from 2013 and 2014 showed no significant difference between the poor 2013 and good 2014 years which implies that when the owls were successful in producing young in 2013 there was sufficient food to keep up with the growth of the pulli. See plot below.

Figure 5 Pulli Weight by age for 2013 and 2014

Figure 5 Pulli Weight by age for 2013 and 2014

Note that the young grow very quickly to a maximum weight at around 6 weeks from hatch and then lose weight towards the usual adult weight of 185 – 350 gms for the female and 275 – 285 gms for the male (Shawyer 1998).

Productivity

Due to the large number of sites and the relatively low number of monitors there are not many records of fledging and the fact that chicks are ringed does not necessary mean that they fledge. A total of 14 chicks from those ringed were found dead in boxes later and some of the smaller pulli in larger broods that were ringed may also not have fledged.  However, early indications show that the number of pulli in each brood was also at its highest level since the project began, with broods of seven being noted in four parishes and eight in another.  In total, 906 pulli were ringed from 1131 that were reared, which gives an overall fledging success rate of 3.77 pulli per nest site.  Boxes will be checked for the remains of dead chicks during the initial box checks this summer, so this figure may be revised accordingly.

It is not unusual for monitors to often come across large clutches, but rarely does this result in a chick fledging from every egg.  In previous years, clutches of 12 and, on one occasion, 13 have been found, but such large clutches are rarely successful.  Why such large broods?  Barn Owls are relatively short-lived species with the longevity record of a wild bird standing at only eight years.  They also have to brazen out poor vole years and perhaps not nest at all that year, so will lay large numbers of eggs in good vole years to compensate.  To help their cause males can be bigamous, especially in good vole years, and will serve two females at the same nest site or at two sites close to each other.  This could explain the apparent large clutches at single nest sites.  The males carry out all the hunting whilst the female incubates and needs to catch enough prey for her to sustain her body weight.  Also, on some occasions, females may incubate a clutch of infertile eggs and then lay another clutch that may also be sterile, which again would explain unusually large broods.   Seven pulli fledging from a single nest is the most ever recorded in Suffolk up to 2014, but a brood of eight which fledged from a box at Redgrave and Lopham Fen beat that record.  Broods of seven recorded in 2014 at Nayland, Assington, Flatford and Wickhambrook are also worthy of note.

Movement

Most of the Barn Owl pulli and some adults monitored by the project are ringed. This provides us with an insight into their movements.   Young Barn Owls are known to disperse from their natal areas soon after fledging and studies have shown that 37% of pulli ringed move more than three kilometres from their nest sites within two weeks of fledging (Bunn et al 1982).  In 2014, 42 of the 68 adult barn owls handled were already carrying rings, giving the Project a series of family trees as well as tracking their movements between boxes.

The known movements of all Barn Owls are shown in figure 6 below

Figure 6 Movement of All Ringed Barn Owls in Project

Figure 6 Movement of All Ringed Barn Owls in Project

And the Barn Owls that were ringed as pulli and then recovered dead are shown in figure 7.

Figure 7 Movement of Barn Owls Ringed as Pulli and found Dead

Figure 7 Movement of Barn Owls Ringed as Pulli and found Dead

 

Patrick Barker holding two barn owl pulli referred to below. Photographer: Katie Smith

Patrick Barker holding two barn owl pulli referred to below. Photographer: Katie Smith

One notable movement involved a Suffolk-ringed chick in 2013.  Project Area Coordinator, Patrick Barker, ringed a brood of two pulli at Lea Farm, Great Ashfield, near Stowmarket  on 7th August 2013 and one of these (a female) was found incubating a clutch of eggs in a box in Muston,  North Leicestershire on the 9th May 2014, a distance of 136 km.  She raised three pulli in her newly adopted county.  Jim Lennon, of South Notts Ringing Group (who monitor boxes in North Leicestershire on behalf of the Vale Barn Owls Project), said “This was the first time in five years that the box has been used; we had several instances of this in 2014. The female was not moulting when caught, and we checked for a second breeding attempt which did not happen in that box, but chicks seem to have got away okay”

 

References:

Bunn, D. S., Warburton, A. B. & Wilson, R. D. S. 1982. The Barn Owl. T & A.D. Poyser, Carlton.

Hillier, A. 2012. Stemming the decline in an iconic species – Suffolk Community Barn Owl Project  results.  The Harrier,

                170: 3-9

Shawyer, C. 1998. The Barn Owl. Arlequin Press

Wright, M. T.  2001. Survey of Breeding Raptors and Owls in Suffolk 1995-1998. Suffolk Ornithologists’ Group, Ipswich.

 

 

Steve Piotrowski and Alec Hillier

16th February 2015