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2.3 Breeding Bird Survey

In 1994 the BTO/JNCC/RSPB Breeding Bird Survey (BBS) was launched following two years of extensive pilot work and earlier desk-based studies. The introduction of the BBS was a response to the limitations of the Common Birds Census (CBC), which has monitored bird populations since 1962. It was recognised that there was a need to improve the geographical representation of UK bird monitoring and, thereby, both species and habitat coverage. The BBS uses line transects rather than the time-consuming territory-mapping method used by the CBC. This makes the survey relatively quick and convenient to undertake, and has been successful in encouraging a large number of volunteers to take part.

The sampling units are 1 x 1 km squares of the National Grid. They are selected randomly by computer (see Data Analysis below). The BBS requires a relatively large sample of survey squares and the aim is to achieve coverage of about 2500 squares in the UK. An important aspect of BBS is its coordination through a network of BBS Regional Organisers, who are also volunteers. Information and survey forms are distributed to organisers, who contact volunteers willing to survey the squares every year, and after the field season, forms are returned to BTO headquarters via the Regional Organisers.

Fieldwork involves three visits to each survey square each year. The first is to record details of habitat and to establish the survey route, the second and third to count birds. A survey route is made up of two roughly parallel lines, each 1 km in length, although for practical reasons routes typically deviate somewhat from the ideal. Each of these lines is divided into five sections, making a total of ten 200 m sections, and birds and habitats are recorded within these units. The two bird-count visits are made about four weeks apart (ideally early May and early June), ensuring that late migrants are recorded. Volunteers record all the birds they see or hear as they walk along their transect routes. Birds are noted in three distance categories (within 25 m, 25-100 m, or more than 100 m on either side of the line) measured at right angles to the transect line, or as in flight. Recording birds within distance bands is important because it provides a measure of bird detectability in different habitats and allows population densities to be estimated. The total numbers of each species, excluding juveniles, in each 200 m transect section and distance category, are recorded on summary forms, as well as the timing of the survey and weather conditions. The average time observers spend per visit is around 90 minutes.

In the first year (1994), 1569 plots were surveyed. The number has increased steadily from 1751 in 1995, 1919 in 1996, 2194 in 1997 and 2310 in 1998 to 2379 in 1999, close to the original target of 2500. Only around a quarter of these plots were covered in 2001, however, owing to Foot & Mouth Disease access restrictions. Squares are distributed throughout the UK, and cover a broader range of habitats than the CBC, including uplands and urban areas. In 1999, 217 species were recorded, 88 from more than 100 squares and a further 13 species from 50-100 squares. For a small number of species, which are colonial or flocking in habit, it is unclear how well they are monitored by the BBS but they are not currently monitored by other BTO schemes, and have therefore been included.

Data Analysis
Survey squares are chosen randomly using a stratified random sampling approach from within 83 sampling regions, which in most cases are the standard BTO regions, based on membership distribution. "Stratified random" means that the country is divided up into regions ("strata") within each of which a certain number of survey squares are chosen at random. BBS regions with larger numbers of potential volunteers are allocated a larger number of squares, enabling more birdwatchers to become involved in these areas. This does not introduce bias into the results because the analysis takes annual differences in the coverage of each region into account.

Change measures between years are assessed using a log-linear model with Poisson error terms. For each species, the higher count from the early or late counts for each square is used in the model (or the single count if the square was visited only once). Counts are modelled as a function of square and year effects. Each observation is weighted by the number of 1-km squares in each region divided by the number of squares counted in that region, to correct for the under- or over-sampling of BBS regions within the UK. The upper and lower confidence limits of the changes indicate the certainty that can be attached to each change measure. When the limits are both positive or both negative, we can be 95% confident that a real change has taken place.

Trends are presented as graphs in which annual estimates are shown in blue and their 95% confidence limits in green. A caveat of "Small sample" is provided where the mean sample size is less than 50 plots per year.

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This report should be cited as:
Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Beaven, L.P., Downie, I.S., Freeman, S.N., Leech, D.I., Marchant, J.H.,
Noble, D.G., Raven, M.J., Simpkin, A.P., Thewlis, R.M. and Wernham, C.V.
(2002) Breeding Birds in the Wider
Countryside: their conservation status 2001. BTO Research Report No. 278. BTO, Thetford. (

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