The BTO/JNCC/RSPB Breeding Bird Survey (BBS) was launched in 1994, following two years of extensive pilot work and earlier desk-based studies. The introduction of the BBS was a move designed to overcome the limitations of the Common Birds Census (CBC), which had monitored bird populations since 1962. In particular, it improves the geographical spread of UK bird monitoring, thus boosting coverage of species and of habitats.

The BBS uses line transects rather than the more intensive territory-mapping method that had been used by the CBC. The average time observers spend per visit on counting birds is only around 90 minutes and, even with travel and data-input time, this survey is relatively quick to undertake and is therefore accessible to a large number of volunteers.

Sampling units are the 1x1-km squares of the Ordnance Survey national grid, of which there are some 254,000 in the UK. From these we make random selections for inclusion in the scheme (see Square selection, below). The BBS requires a relatively large sample of survey squares, and the initial aim was to achieve coverage of about 2,500 squares (1%). This total is now well exceeded.

An important aspect of BBS is its coordination through a network of volunteer BBS Regional Organisers. The Regional Organisers find and encourage willing volunteers for their squares and provide paper forms as required. Since 2003, when online submission of BBS data was introduced, most data have been returned online – see the BBS pages of the main BTO website for details.

Fieldwork involves up to three visits to each survey square each year. The first is to record details of habitat and to establish or re-check the survey route, while the second and third (termed 'early' and 'late') are to count birds. A survey route is composed 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 ten units. The two bird-count visits are made about four weeks apart (ideally in early May and early June), ensuring that late-arriving 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 provides a measure of bird detectability in different habitats and thus allows population densities to be estimated more accurately. The total numbers of each species, excluding juveniles, are recorded in each 200-m transect section and distance category, as well as the timing of the survey and weather conditions. 

In 2014, the optional recording of the method of detection was included in BBS for the first time, and observers can now record whether they detect each individual bird by sight, by song or by call. This information is not currently used to calculate trends, but it is anticipated that it will help further refine the calculation of population densities for some species.

By 1998, more than 2,300 BBS squares were being surveyed annually, close to the original target of 2,500. Only around a quarter of these plots were covered in 2001, owing to Foot & Mouth Disease access restrictions, but (thanks to our keen observers) the sample recovered immediately.

The original target of 2,500 was surpassed in 2004 and coverage had increased further to over 3,700 in 2007, running marginally below that level over the next few years during and just after the 2007–11 Bird Atlas, before increasing again, with over 4,000 squares being covered for the first time in 2018 (Harris et al. 2019). The survey was affected by the COVID-19 lockdown in 2020 and full results could not be produced, with coverage in 2021 down slightly on 2019 at just over 3,900 squares. 

Although data from 2001 and 2020 are excluded from the analysis, due to the effects of Foot & Mouth Disease and COVID-19 respectively, population trends which include both years can still be produced as the smoothed trend using data for all other years enables a robust population estimate to be made for these missing years.

Squares are distributed throughout the UK and cover a broad range of habitats, including uplands and urban areas. There are now 117 species that are present on 40 or more BBS squares annually and so can be monitored with good precision at the UK scale (Joys et al. 2003, Harris et al. 2020), although a few present special difficulties because of their colonial or flocking habit or their wide-ranging behaviour.

For most of these species, BBS can also assess annual population changes within England alone, using data from 30 or more squares, and for about half the species also within Scotland and Wales as separate units. Sample sizes in Northern Ireland already allow more than 30 species to be indexed annually. 

In the 2017 BBS report (Harris et al. 2018), shorter-term 5-year and 10-year trends were shown for the first time in addition to the trends covering the full BBS period since 1994. For five of the 117 species which can be monitored by BBS, results can only be produced over these shorter time periods, due to the lower sample sizes in the early years of the survey.

Square selection

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. Survey squares are chosen at random within each region, to a density that varies with the number of BTO members resident there.

Regions with larger numbers of potential volunteers are thereby allotted 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 the regional differences in sampling density into account.

Data analysis

Change measures between years are assessed using a log–linear model with Poisson error terms. For each species and square, counts are summed across all sections and distance bands for each visit ('early' and 'late') and the higher value 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 there, to correct for the differences in sampling density between regions. 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 85% confident that a real change has taken place (see the 'confidence' section for details).

Trends are presented as graphs in which annual population indices are shown alongside a smoothed trend and its 85% confidence limits. A caveat, 'small sample', is provided against the trends for England, Northern Ireland, Wales and Scotland where the mean sample size is between 30 and 40 plots per year. 

Go to the BBS section of the BTO website.