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Project Barn Owl

Barn Owl © Mike TomsThe need for a replicable baseline estimate of the Barn Owl population prompted a collaboration between the BTO and the Hawk and Owl Trust, called Project Barn Owl. The project recognised the importance of such an estimate in allowing future changes in population levels to be monitored and for causal factors to be identified and later quantified.

Volunteer fieldworkers were asked to visit 2x2 km survey squares (or tetrads) during the winter to record all potential nesting sites. Such sites included farm buildings, cavities in trees, bale stacks and nest boxes. During the summer these sites were rechecked for signs of occupancy. 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 our fieldworkers spent an estimated 11,000 hours on fieldwork (averaging a staggering 30 hours each), covering 81% of the tetrads in at least one of the years. Confirmed breeding attempts were recorded in 82 tetrads with a total of 133 breeding attempts over the three years. The number of confirmed breeding pairs in each survey stratum were used to produce a national estimate for each survey year, suggesting a baseline population estimate of c.4,000 breeding pairs with 95% confidence intervals of 1,000 pairs.

Regional and temporal variations in the size of the breeding population paralleled similar variations in breeding performance as determined from nest record cards. Estimates of regional density from the current survey were generally lower than the corresponding estimates from the previous survey carried out by the Hawk and Owl Trust (1982-1985). 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.

Another aspect of Project Barn Owl examined the feasibility of establishing an annual monitoring programme (now set up as the Barn Owl Monitoring Programme), with detailed information being gathered on site occupancy, productivity and survival on an annual basis. This is really an extension of the 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. The success of Project Barn Owl was due to the hard work and dedication of the many fieldworkers and local co-ordinators, 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.