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2.3 Combined Common Birds Census (CBC)
and Breeding Bird Survey (BBS) trends
The field protocols for the two surveys are described in sections
2.1 and 2.2. As previously noted, the CBC
has been an enormously influential project, providing the main source
of information on national population levels in the UK since its
inception in 1962. Coverage was predominantly in lowland England,
where the numbers of potential volunteers are greatest, while coverage
was more patchy in more sparsely populated regions and especially
the uplands (Marchant et
al. 1990). CBC plots were situated in a limited number
of habitats, predominantly farmland and woodland. Within a large
rectangle of southeastern Britain (covering England and Wales south
and east from Seascale, Scarborough and Exeter), the plots are nevertheless
believed to be broadly representative, at least of lowland landclasses
(Fuller et al. 1985).
For species such as Wood Warbler and Meadow Pipit that have the
greater part of their numbers in the far west or north of Britain,
however, the CBC may not have accurately reflected UK trends.
The BBS, on account of its more rigorous,
stratified random sampling design, and its simplicity in the field,
produces data that better cover the previously underrepresented
regions and habitats. In some early
editions of 'Breeding Birds in the Wider Countryside’
(e.g. Baillie et al.
2002), separate indices were published from CBC and BBS data,
for those species with sufficiently large sample sizes. There being
no new CBC data since 2000, however, it is unnecessary to present
a CBConly trend – except for those few species that are now
so rare that no joint or BBS index is available.
For most purposes, the presentation and analysis of longer timeseries
is required, dating back to before the establishment of the BBS
but coming right up to the present day. The calculation of 25year
alert designations, as in this report, provides just one example.
This need led the BTO to research the compatibility of indices from
BBS and CBC data in various years and regions, and the possibility
of deriving trustworthy longterm indices from the two data sources
in combination (Freeman
et al. 2003, 2007a).
This research suggested that for the vast majority of species considered
there was no significant difference between population trends, calculated
from the two surveys, based on that part of the country where CBC
data are sufficient to support a meaningful comparison. Where a
statistically significant difference was found, this was sometimes
for very abundant species for which the power to detect even a biologically
insubstantial difference was considerable. Within this region, therefore,
longterm trends based on CBC and BBS data can be produced for almost
all species previously monitored by the CBC alone. For (Freeman
et al. 2003, 2007a)
this was the area covered by Fuller
et al. (1985), because CBC plots in that region were
shown to be representative of lowland farmland there. As this region
covers the bulk of England, and for consistency with the rest of
this report, we have produced joint indices for CBC/BBS for the
whole of England (the CBC/BBS England index), rather than just the
English part of the 'Fuller rectangle'.
A second question then is whether one can obtain reliable trends
over the same period for the entire UK. That is, since prior to
1994 only CBC data are available, are the population trends within
the region well covered by the CBC typical of those for the UK as
a whole? The shortage of CBC data in the north and west means that
the only way of investigating this is via the BBS data. Significant
differences in trends between the area well covered by the CBC and
the rest of the UK were found for approximately half the species
(see Freeman
et al. 2003, 2007a,
for full details). For such species, a regional bias in CBC data
means that no reliable UK index can be produced prior to 1994. In
summary, joint population indices dating back to the start of the
CBC can continue to be produced for that part of the country well
served by the CBC (essentially England) for almost all common species.
However, a similar UK index can be produced for only about 50% of
species (CBC/BBS UK index).
This report presents joint CBC/BBS trends for the UK and/or England,
as appropriate. Ideally the trends would have been estimated using
generalised additive models (Fewster
et al. 2000) but these were too computationally intensive,
given the large number of sites involved. Therefore we fitted a
generalised linear model, with counts assumed to follow a Poisson
distribution, and a logarithmic link function, to the combined CBC/BBS
data. Standard errors were calculated via a bootstrapping procedure
and there is therefore no need to model overdispersion, as it does
not affect the parameter estimates. BBS squares were weighted by
the number of 1km squares in each sampling region divided by the
number of squares counted in that region as in standard BBS trend
analyses. CBC plots were assigned the average weight of all BBS
squares as this allows them to be incorporated within the analysis
while retaining the convention of not applying weights within the
BBS sample. The population trend was smoothed using a thinplate
smoothing spline with degrees of freedom about one third the number
of years. Confidence intervals were calculated via a bootstrap procedure.
Bootstrap samples were generated by resampling sites from the original
data set, with replacement. A generalised linear model was then
fitted to each bootstrap replicate and a smoothing spline fitted
to the annual population indices as described above. Confidence
limits were then calculated as the appropriate percentiles from
the sets of smoothed estimates. The overall result is a smoothed
trend that is mathematically equivalent to that produced from a
generalised additive model. The method of estimation is less statistically
efficient because the smoothing is not incorporated within the estimation
procedure, and is likely to have resulted in more conservative statistical
tests and wider confidence limits. However this compromise was necessary
to make it possible to fit the trends within a reasonable amount
of computer time (still several weeks).
Indices are
plotted as the blue line on the graphs, and provide a relative measure
of population size on an arithmetic scale relative to an arbitrary
value of 100 in one of the recent years of the sequence. If an index
value increases from 100 to 200, the population has doubled; if
it declines from 100 to 50, it has halved. Note that positive and
negative percentage changes are not directly equivalent: for example,
a decrease of 20% would require an increase of 25% to restore the
population to its former level. 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, and 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: nonoverlap
of the 85% confidence limits is equivalent to a significant difference
at approximately the 5% level (Anganuzzi
1993).
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