Project Barn Owl
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
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.