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Common Birds Census
The Common Birds
Census (CBC), which began in 1962, was the first of the BTO's monitoring
schemes for widespread breeding birds, but has now been superseded
for this purpose by BBS. The 2000 field season
was the CBC's last year of full operation. Since 2001, a reduced
set of CBC volunteers have continued to survey their plots, under
the 'Core Mapping Census', with the aims of providing information
on the relationships between territory locations and habitat features,
and of monitoring birds in a small number of specific habitat types.
of the CBC as a monitor of UK bird populations were largely related
to the time-consuming nature of both fieldwork and analysis. This
inevitably limited the number of volunteers able to participate
in the scheme, with the result that areas with few birdwatchers
were under-represented. Constrained by the relatively small sample
size, CBC concentrated on farmland and woodland habitats. Bird population
trends in built-up areas and the uplands were therefore poorly represented.
Moreover, as the plots were chosen by the observers, some may not
have been representative of the surrounding countryside and there
may be some bias towards bird-rich habitats. It is for these reasons
that the BBS was introduced in 1994. Both surveys were run in parallel
for several years to allow calibration between the schemes.
now provides monitoring that is more representative of UK populations
and covers more species, only CBC covers the mid 1960s to mid 1990s,
a period of great change in bird populations. CBC results have been
hugely influential in determining conservation priorities in the
UK countryside. The store of detailed maps of almost a million birds'
territories, collected through the CBC and maintained by BTO since
the early 1960s, is a mine of information of unique value for investigating
the relationships between breeding birds and their environment,
over wide temporal and spatial scales, and is increasingly used
for research. For many species, CBC and BBS trends can be linked
to form joint CBC/BBS trends that provide ongoing monitoring, continuous
since the 1960s.
for the CBC mapping method are still freely available by post from
BTO headquarters and will shortly be available through the BTO website.
The method is recommended where detailed information on territory
numbers and locations is needed for a site and sufficient time is
available for a thorough survey.
The results from the Common Birds Census (CBC) provided reliable
population trends for more than 60 of the commoner UK breeding species.
The CBC was
instigated to provide sound information on farmland bird populations
in the face of rapid changes in agricultural practice. Fieldwork
was carried out by a team of 250-300 dedicated volunteers. The same
observers surveyed the same plots using the same methods year after
year. On average, plots were censused for around seven consecutive
years but a few observers surveyed the same sites for more than
30 years. Although the original emphasis was on farmland plots,
woodland plots were added by 1964. Farmland plots averaged around
70 hectares in extent. Woodland plots were generally smaller, averaging
just over 20 hectares. A small number of plots of other habitats,
including heathlands and small wetlands, were also surveyed annually,
especially before 1985.
approach was used to estimate the number and positions of territories
of each species present on each survey plot during the breeding
season. Volunteers visited their survey plots typically eight to
ten times between late March and early July and all contacts with
birds, either by sight or sound, were plotted on 1:2500 maps. Codes
were used to note each bird's species, with sex and age where possible,
and also to record activity such as song or nest-building. The registrations
were then transferred to species maps and returned to BTO headquarters
for analysis. Observers also provided maps and other details of
the habitat on their plots. This makes it possible to match the
distribution of bird territories with habitat features, providing
the potential for detailed studies of bird-habitat relationships.
of registrations on the species maps reveals the numbers of territories
for each species. All assessments of territory number were made
by trained BTO staff, applying rigorous guidelines, to ensure consistency
between estimates across sites and years.
In 1990, the
results from the Common Birds Census were brought together in the
book Population Trends in British Breeding Birds (Marchant
et al. 1990). This landmark publication discussed
long-term population trends for the years 1962 to 1988 for 164 species,
with CBC or WBS population graphs for around two-thirds of these.
The CBC was the first national breeding bird monitoring scheme of
its kind anywhere in the world and its value has been widely recognised
internationally. The territory-mapping method adopted by the CBC
is acknowledged as the most efficient way of estimating breeding
bird numbers in small areas. As the benchmark to which other survey
methods are compared, it is important that the validity and limitations
of the CBC methods are understood. Snow
(1965) compared CBC mapping and intensive nest-finding,
and concluded that mapping censuses are good indicators of breeding
population size for 70% of species. Experiments to test differences
between observers' abilities to detect birds found that, although
there was considerable variation between individual abilities, the
observers were consistent from year to year (O'Connor
& Marchant 1981). As the CBC relies on data from plots covered
by the same observer in consecutive years, this source of bias has
no implications for the CBC's ability to identify population trends.
It has also been confirmed that the sample of plots from which CBC
results are drawn has not changed in composition or character over
the years (Marchant et
al. 1990) and that the results of territory analysis
are not affected by changes in analysts, once trained (O'Connor
& Marchant 1981). Fuller
et al. (1985) found that farmland CBC plots were
representative of ITE lowland land-classes throughout England (excluding
the extreme north and southwest), and closely reflected the agricultural
statistics for southern and eastern Britain.
Few trend graphs in this report are drawn solely from CBC data.
The following information will aid their interpretation.
are modelled using a generalised additive model (GAM), a type of
log-linear regression model that incorporates a smoothing function
(Fewster et al.
2000). This replaces the Mountford model that employed a
6-year moving window (Mountford
Peach & Baillie 1994)
and was used until 1999, but the principles are similar. Counts
are modelled as the product of site and year effects on the assumption
that between-year changes are homogeneous across plots. "Smoothing"
is used to remove short-term fluctuations (e.g. those caused by
periods of severe weather or measurement error) and thus reveal
the underlying pattern of population change. This is achieved by
setting the degrees of freedom to one-third the number of years
in the series. Confidence limits on the indices are estimated by
bootstrapping (a resampling method; Manley
1991) and thus do not make any assumptions about the underlying
distribution of counts.
plotted as the blue line on the graphs, and provide a relative measure
of population size on an arithmetic scale with a 1998 value of 100.
If an index value increases from 100 to 200, the population has
doubled; if it declines from 100 to 50, it has halved. The two green
lines on the graphs, above and below the index line, are the upper
and lower 85% confidence limits. A narrow confidence interval indicates
that the index series is estimated precisely, a wider interval indicates
that it is less precise. The use of 85% confidence limits allows
relatively straightforward comparison of points along the modelled
line: non-overlap of the 85% confidence limits is equivalent to
a significant difference at approximately the 5% level (Anganuzzi
1993). Caveats are provided to show where the data suffer
from a "Small sample" if the mean number of plots was
<20. Data are regarded as "Unrepresentative?" if the
average abundance of a species in 10-km squares containing CBC plots
was less than that in other 10-km squares of the species' distribution
in the UK (as measured from 1988-91 Breeding Atlas data
(Gibbons et al.
1993)), or, where average abundances could not be calculated,
expert opinion judged that CBC data may not be representative.
Page - 2.3 Joint CBC/BBS trends
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