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2.8
The alert system
2.8.1
General approach
The alert system
used within this report is designed to draw attention to developing
population declines that may be of conservation concern, and is
described in detail by Baillie
& Rehfisch (2006). It also identifies situations where longterm
declines have reversed, leading to an improvement in conservation
status. It must be stressed that the changes reported here are advisory
and do not supersede the agreed UK conservation listings (Eaton
et al. 2009; see PSoB
pages). They are based on similar criteria to Birds of Conservation
Concern, however, and so provide an indication of likely changes
at future revisions.
The system is
based on statistical analyses of the population trend data for individual
species. Alerts seek to identify rapid declines (>50%) and moderate
declines (>25% but <50%). These declines are measured over
a number of timescales, depending on the availability of data –
the full length of the available time series, and the most recent
25 years, 10 years and 5 years for which change can be estimated.
The conservation emphasis is particularly on the longer periods,
but shortterm changes help to separate declines that are continuing
– or accelerating – from those that have ceased or reversed.
The alerts are
calculated annually using standard automated procedures. Where species
are at the margin of two categories (e.g. a decline of about 25%)
they may raise alerts in some years but not others, or different
levels of alert in different years.
Data on some
species might be biased, owing to possibly unrepresentative monitoring,
or imprecise, owing to small sample sizes. Because these data often
provide the only information that is available, our general approach
is to report all the alerts raised but to flag up clearly any deficiencies
in the data.
2.8.2
Smoothing population trends
Bird populations
show longterm changes that do not follow simple mathematical trajectories.
In addition to the longterm trends, annual population indices also
show shortterm fluctuations resulting from a combination of natural
population variability and statistical error. We use smoothing techniques
that aim to extract the longterm pattern of population change,
without forcing it to follow any particular shape (such as a straight
line or a polynomial curve). These methods remove most of the effects
of shortterm fluctuations (including any natural yeartoyear variability)
so that the longterm trend is revealed more clearly.
Technical
details available here
2.8.3
Years used for analysis
Once a smoothed
population trend has been calculated, change measures are calculated
from the ratio of the smoothed population indices for the two years
of interest. Population indices for the first and last years of
a smoothed time series are less reliable than the others, and so
we always drop them before calculating alerts. Because the latest
year is not included, the alerts are therefore less uptodate than
they could be, but fewer false alarms are generated. The latest
year's data points do contribute, however, to the smoothed curve
and are dropped only after the smoothing has taken place.
The time it
takes BTO to collate and analyse each year's intake of bird monitoring
data is another factor affecting the years that can be included
in these analyses. Full analyses of data sets are not usually all
available until 12–15 months after the end of a particular
breeding season. Thus for a report prepared in year x (e.g.
2010) we have analyses of monitoring data up to year (x1)
(e.g. 2009). As we drop the final year of the smoothed time series,
we report here on change measures up to year (x2) (e.g.
2008).
Longterm changes
for most of the species included in this report are calculated from
joint Common Birds Census and Breeding Bird Survey data (CBC/BBS
indices). The CBC started on farmland in 1962 and on woodland in
1964. However, the early years of the CBC population indices are
strongly influenced by the effects of the unusually severe winters
of 1961/62 and 1962/63, as well as by developments in methodology
(Marchant et al.
1990). Joint CBC/BBS indices have been calculated using
only the data from 1966 onwards, therefore, and population changes
are calculated back to 1967.
2.8.4
Confidence limits and statistical testing
We show 90%
confidence limits for population change measures wherever possible.
Any decline where the confidence limits do not overlap zero (no
change) is regarded as statistically significant and will trigger
an alert if it is of sufficient magnitude. Note that, because we
are seeking to detect only declines, we are using a onetailed test
– with a P value of 0.05. These confidence limits
therefore do not indicate whether increases are statistically significant.
The graphs of
population trends show 85% confidence limits because these allow
an approximate visual test of whether the difference between the
index values for any two given years is statistically significant:
if the index values for two given years are assumed to be independent,
and normally distributed with standard errors of comparable size
(standard errors differing by a factor of up to about 2 are quite
acceptable), then to a good approximation the difference between
them is significant at the 5% level if there is no overlap in their
85% confidence intervals (Buckland
et al. 1992, Anganuzzi
1993). This test is fairly robust, and the independence assumption
is reasonable if the years are well separated.
Technical
details available here
2.8.5
Datadeficient species
There is uncertainty
about the reliability of the results for some species, either because
data may be unrepresentative or because they are based on a very
small sample of plots. In these cases the cause of the uncertainty
is recorded in the comment column of the population change table.
Unrepresentative
data
In this report
we present joint UK or England CBC/BBS trends only if there was
no substantial or statistical difference between the trends from
the two schemes over the period when they ran in parallel (Freeman
et al. 2007a). Thus, since BBS results are drawn from
a random sample, the trends are always considered to be representative
of the region concerned.
In previous
reports representativeness was assessed using the criteria developed
by Gibbons et al.
(1993). Data from the 1988–91 Breeding Atlas were
used to compare the average abundance of a given species in 10km
squares with and without CBC plots. If average abundance is higher
in squares without CBC plots, it is likely that much of the population
is not well sampled by the CBC. In past reports, CBC data for such
species were labelled as "unrepresentative". Where there
are insufficient data to undertake such calculations, expert opinion
was used instead.
Sample
size
Sample size
is assessed from the average number of plots contributing to the
population indices for a given species in each year. A plot with
a zero count would be included provided that the species had been
recorded there in at least one year and that records for that plot
were available for at least two years. Plots where a species has
never been recorded do not enter the index calculations. These average
sample sizes are shown in column four ('plots') of the population
change tables. For CBC, WBS and CES, a mean of between 10 and 19
plots is flagged as a small sample. For BBS indices for individual
countries a mean in the range 30–39 plots is flagged as a
small sample. UK BBS indices are presented only where samples reach
at least 40 plots.
Technical
details available here
Next section – 2.9 Statistical methods
used for alerts
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