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Breeding Bird Survey
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 representativeness of UK bird monitoring,
and thereby promotes both species and habitat coverage. The BBS
uses line transects rather than the more intensive territory-mapping
method used by the CBC. This makes the survey relatively quick to
undertake, and has been successful in encouraging a large number
of volunteers to take part. The average time observers spend per
visit is only around 90 minutes.
units are 1x1-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 volunteer BBS Regional Organisers. Information and survey forms
are distributed first to these organisers, who contact volunteers
willing to survey the squares every year. After the field season,
forms are returned to BTO headquarters again via the Regional Organisers.
On-line submission of BBS data is now also available and is recommended
- see the BBS pages
of the main BTO website for details.
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 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
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 is
important because it provides a measure of bird detectability in
different habitats and 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 UK weather conditions.
By 1998, more than 2300 BBS squares were being surveyed annually,
close to the original target of 2500. 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 to over 2100 in 2002. Squares are distributed throughout
the UK and cover a broad range of habitats, including uplands and
urban areas. Around 105 species are present on 40 or more BBS squares
annually and can be monitored with good precision at the UK scale,
although a few present special difficulties because of their colonial
or flocking habit or their wide-ranging behaviour. For most of these,
BBS can also assess annual population changes within England alone,
and for about half the species also within Scotland and Wales as
separate units. Sample sizes in Northern Ireland currently allow
about 20 species to be indexed annually.
Survey squares are chosen randomly using a stratified random sampling
approach from within 83 sampling regions. These sampling regions,
which in most cases are the standard BTO regions, are the "strata"
(literally layers) of the sample. Survey squares are chosen at random
within each region (stratum), to a density that varies with the
number of BTO members resident there. Regions with larger numbers
of potential volunteers are thereby 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 the differences in regional sampling density into
between years are assessed using a log-linear model with Poisson
error terms. For each species, the higher count from the total 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 differences
in sampling density 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 in the range 30-39 plots
per year for England, Northern Ireland, Wales and Scotland trends.
A minimum sample size of 40 plots was required for the UK.
section - 2.2 Common Birds Census
to Methodology Index
CLICK HERE to go
to the BBS section of the main BTO website