Barn Owl Populations and Monitoring  
What we learned from "Project Barn Owl"

The long-term decline of Barn Owls, together with the ongoing conservation efforts targeted towards this species, long highlighted the need for a suitable baseline estimate of the Barn Owl population within the UK. Such an estimate is particularly important if we are to monitor future changes in the Barn Owl population.

Project Barn Owl, a collaboration between the British Trust for Ornithology (BTO) and the Hawk and Owl Trust (HOT), was set up to produce this baseline estimate, and to produce it using a methodology that could be repeated in future years.

Following a pilot year a suitable methodology was developed. This involved fieldwork within some 1100 survey tetrads (2 x 2km squares) that had been selected in a stratified random manner, based on the previous occurrence of breeding Barn Owls. In each of the survey years, volunteer fieldworkers were asked to survey their tetrads during the winter months to record all potential nesting sites. Such sites included farm buildings, cavities in trees, bale stacks and nest boxes. During the summer months these sites were rechecked for signs of occupancy and evidence of breeding Barn Owls (all fieldworkers were licensed by the relevant statutory country conservation agencies). The fieldwork was carried out over three years (1995 - 1997) to allow for the natural variation in breeding attempts resulting from changes in prey availability and short-term climatic events.

Over the course of the three years fieldworkers spent an estimated 11,000 hours on fieldwork (averaging a staggering 30 hours each), and covering 81% of the tetrads in at least one of the years. Although the amount of time spent on fieldwork varied regionally (because some parts of the country were more difficult to survey than others) there were no differences between the three strata used and, more importantly, no differences in terms of the time spent on fieldwork between those who found owls and those who did not. A random sample of the tetrads (c. 6% of the total) were visited by the Project Officer for the purposes of validation. This showed that no breeding Barn Owls were missed by fieldworkers and that 92% of those potential nest sites defined as being suitable for a breeding Barn Owl were recorded successfully. This demonstrated quite clearly that fieldworkers achieved a high degree of survey reliability.

Confirmed breeding attempts were recorded in 82 tetrads with a total of 133 breeding attempts recorded over the three years. The number of confirmed breeding pairs in each stratum were used to produce a national estimate for each survey year (Table 1), suggesting a baseline population estimate of c. 4,000 breeding pairs with confidence intervals of plus or minus 1,000 pairs.

Year Estimate(breeding pairs) 95% Confidence Intervals
1995 2830 1951-3761
1996 3967 2785-5252
1997 3951 2769-5214
Regional and temporal variations in the size of the breeding population paralleled similar variations in breeding performance as determined from BTO nest record cards (Figure 1 and Table 2). Estimates of regional density from the current survey were generally lower than the corresponding estimates from the previous survey carried out by Colin Shawyer (1982-1985)and published by the Hawk and Owl Trust. Only for East Anglia was the current density estimate notably higher, indicating that there has been a real increase in Barn Owl densities in this region since the mid-1980s.

Barn Owls breeding in south western England appear to be the most productive, with large clutch and brood sizes and high nesting success, whereas Barn Owls in Wales and northern England are the least productive with generally small clutch and brood sizes.

Project Barn Owl also provides the first replicable dataset on the availability of the potential nest sites for Barn Owls on a regional basis.

On average, tetrads contained 7-8 potential nest sites, most commonly farm buildings and tree cavities (notably in Oak and Ash).

The availability of nest boxes was significantly greater in tetrads containing breeding Barn Owls than in those with no owls. This could suggest one of two things: either that the increased availability of nestboxes in an area increases the likelihood of Barn Owls breeding in the square; or that people deliberately erect nestboxes in those areas where Barn Owls already occur.

In reality, both situations probably happen. The data on potential nest sites also demonstrated that Barn Owls nest more frequently in buildings in the (wetter) west than in the east of the UK because of a lack of suitable tree sites, and not because of any direct preference for buildings.

Another aspect of Project Barn Owl examined the feasibility of establishing an annual monitoring programme, with detailed information being gathered on site occupancy, productivity and survival on an annual basis. This is really an extension of the BTO’s Nest Record Scheme approach, with more detailed information being collected making it easier to understand how environmental factors influence the population dynamics of this species.

The combination of a reliable baseline estimate and an annual monitoring programme makes it possible to gather the kind of data needed if we are to understand population change in the Barn Owl and make predictions for the future. If we want to know how future changes in land use will influence the Barn Owl population then we need to understand which factors are important now, so that we can predict which will be important in the future.

The success of Project Barn Owl was due to the hard work and dedication of the many fieldworkers and local coordinators, to whom we are very grateful. We are also especially grateful for the generous sponsorship of Bayer AG, LIPHA SA, Sorex Ltd and Zeneca Agrochemicals.

A small number of copies of the full technical report are now held by HOT and BTO, which are available on loan. This technical report is not intended for wider circulation because it contains confidential statistical information.

Mike Toms, Project Coordinator.
Colin Shawyer, Project Supervisor.
Humphrey Crick, Project Supervisor.

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