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2.7 The Alert System

2.7.1 General approach
2.7.2 Smoothing population trends
2.7.3 Years used for analysis
2.7.4 Confidence limits and statistical testing
2.7.5 Data deficient species
2.7.6 Alert criteria
2.7.7 Application to individual schemes
2.7.8 Breeding Bird Survey

2.7.1 General approach

The alerting system used within this website is designed to draw attention to developing population declines that may be of conservation concern. It also identifies situations where long-term declines have been reversed leading to an improvement in the conservation status of the species concerned. It must be stressed that the changes reported here are advisory and do not represent a revision of agreed conservation listings (e.g. JNCC's Birds of Conservation Importance list (JNCC 1996) or the NGO Birds of Conservation Concern List (Gibbons et al. 1996)). However, they are based on similar criteria to the formal lists so they provide a good 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%-49%). These declines are measured over the full length of the available time series, 25 years, 10 years and 5 years. The conservation emphasis is particularly on the longer time periods but short term changes help separate those species where the decline is continuing or even accelerating from those which have declined previously but are now stable.

The alerts presented on this website 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 fire alerts in some years but not others.

Data on some species may be potentially biased due to unrepresentative coverage by monitoring schemes or imprecise due to small sample sizes. Because these data often provide the only available information our general approach is to report all the trends that can be calculated but to clearly flag up deficiencies in the data.

2.7.2 Smoothing population trends

Bird populations show long-term changes that do not follow simple mathematical trajectories. In addition to such long-term trends population indices also show annual fluctuations resulting from a combination of natural population variability and statistical error. We use smoothing techniques that aim to extract the long-term 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 short-term fluctuations so that the long-term trend is revealed more clearly.

Technical details available here

2.7.3 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. Therefore we always drop these years before calculating any alerts. It may seem that that the alerts are therefore less up-to-date than they might be but the advantage is that fewer false alarms will be generated. It is important to stress that the final year of data do contribute to the smoothed curve and that the final point is only dropped after the smoothing has taken place.

The time taken to collate and analyse bird monitoring data is another factor affecting the years that can be included in these analyses. Full analyses of all data sets are not usually available until 12-15 months after the end of a particular breeding season. This report was prepared in the first half of 2002 when we had analyses of monitoring data up to 2000. As we drop the final year of the smoothed time series we are using change measures up to 1999.

Long-term changes for most of the species included in this report are calculated from Common Birds Census (CBC) data. 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 if 1961/62 and 1962/63, as well as by developments in methodology (Marchant et al. 1990). Therefore CBC indices have been calculated using the data from 1966 onwards and population changes are calculated back to 1968.

Data for other schemes generally start as soon as the scheme had reached a sufficient size to produce reliable results. The maximum time periods available from the main schemes contributing to this website are set out in the following table.

Scheme
Time series available
Maximum alert period
First year
Last year
First year
Last year
Number
of years
Common Birds Census
1966
2000
1968
1999
31
Waterways Bird Survey
1974
2000
1975
1999
24
Constant Effort Sites
1983
2000
1984
1999
15
Heronries Census
1928
2000
1929
1999
75
Breeding Bird Survey
1994
2000
-
-
-

The Breeding Bird Survey started in 1994 and has not been running for long enough for it to be worthwhile to apply formal alerts methodology. Six year changes based on annual indices are reported here but we do not flag formal alerts.

2.7.4 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 be used to trigger an alert if it is of sufficient magnitude. Note that because we are only seeking to detect declines we are using a one-tailed test with a P value of 0.05. Therefore these confidence limits should not be use to determine 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 indices for any two given years is statistically significant. These 85% confidence intervals provide us with an heuristic test for population change: if the indices for two given years are assumed 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 the indices is significant at the 5% level if there is no overlap in their 85% confidence intervals (Buckland et al. 1992). This test is fairly robust, and the independence assumption is reasonable if the years are some distance apart.

Technical details available here

2.7.5 Data deficient species

There is uncertainty about the reliability of the results for some species, either because data may be unrepresentative or because that 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

This assessment is based on the criteria developed by Gibbons et al. (1993). Data from the New Breeding Atlas were used to compare the average abundance of a given species in 10-km 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. CBC data for such species are labeled as "unrepresentative". Where there are insufficient data to undertake such calculations expert opinion is used.

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 less than 20 plots is flagged as a small sample. For BBS a mean of less than 50 plots is flagged as a small sample.

2.7.6 Alert criteria

Alerts are flagged in two categories, greater than 50% decline (>50%) and 25%-49% decline (>25%). The change measures used are calculated from smoothed time series, wherever possible based on generalized additive models. After smoothing the first and last years are dropped from the time series as they may be unreliable. Alerts are only flagged if the estimated population change is significantly different from zero (no change) based on bootstrapped 90% confidence limits for the population change measure (a one-tailed test). Where change measures may be unreliable due to unrepresentative data or small sample sizes the alert is still flagged but the potential problem is noted.

Alerts are evaluated over the maximum length of the time series, 25 years, 10 years and 5 years. The maximum lengths of the time series used in this report are 31 years for the Common Birds Census, 24 years for the Waterways Bird Survey, 15 years for the Constant Effort Sites Scheme and 75 years for the Heronries Census.

2.7.7 Application to individual schemes

Currently the full methodology outlined above is applied to results from the Common Birds Census and the Waterways Bird Survey. For the Constant Effort Sites scheme and the Heronries census we present annual indices with confidence limits and the fit a smoothed curve through the annual index values. We do not currently have confidence limits for this smoothed curve. Therefore all alert labels for CES are shown in square brackets. There are no alerts for Grey Heron.

Technical details available here

2.7.8 Breeding Bird Survey

The breeding survey started in 1994 so only six years of data (1994-2000) were available for this report. This is not a long enough time series to apply the smoothing methods and alerts framework outlined above. Therefore we have simply calculated change measures between the first and last years of the BBS time series based on the standard sites x years model that is used to produce the BBS indices each year.


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This report should be cited as:
Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Beaven, L.P., Downie, I.S., Freeman, S.N., Leech, D.I., Marchant, J.H.,
Noble, D.G., Raven, M.J., Simpkin, A.P., Thewlis, R.M. and Wernham, C.V.
(2002) Breeding Birds in the Wider
Countryside: their conservation status 2001. BTO Research Report No. 278. BTO, Thetford. (http://www.bto.org/birdtrends)

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