Accredited official statistics

A Brief Introduction to the Wild Birds Populations Indicator

Updated 12 November 2024

Introduction

The annual Defra Wild Bird Populations National Statistics Release is published every autumn see GOV.UK. The headline indicator of this publication is the ‘All- Species’ indicator, which shows trends in abundance of all species of bird that are native to, and breed in, the UK with a population of more than 500 breeding pairs (300 in England) for which there is sufficient information to calculate a trend.

The indicator measures changes in populations of a range of species, relative to historical numbers, where 1970 is the baseline year. A 5% decline in the overall index means that, on average, the population size of species included in the indicator has decreased by 5%. It does not necessarily mean each species has declined by 5% or that the total number of birds in the UK has declined by 5%. It can also mean that the species evenness in the indicator is decreasing (evenness refers to the degree of uniformity of the species proportions: high evenness occurs when many species have similar abundance, with no single species dominating).

Calculating the all-species wild bird indicator

The creation of the all species wild bird indicator involves 2 steps: (1) the production of annual population indices for the individual species for which there is trend data, and (2) the amalgamation of these individual indices into a single aggregate index.

1. Indices for individual species

These are generated by a statistical analysis of representative sites resurveyed year after year (for example, in the Breeding Bird Survey) or based on annual estimates of total populations (for example, Heronries Survey). The population trends for each species are made comparable by expressing them as indices relative to ‘100’ in the start (baseline) year. Thus each annual index shows relative changes in abundance from the start year: a rise to 200 in the index reflects a doubling in numbers, a decline to 50 reflects a halving.

2. Amalgamating into a single index

The all-species index is calculated as the geometric mean of all the individual species indices (see Gregory et al.), with no weightings so each species has the same relative effect on the indicator. The geometric mean is equal to the nth root of the product of all ‘n’ species indices, where ‘n’ is the number of individual species indices. The geometric mean is used to ensure that a doubling in the population index of one species (for example, 100 to 200) is balanced by a halving (for example, 100 to 50) in the population index of another species. The geometric mean of 200 and 50 is 100 (see Example 1, Annex 1).

What does the indicator show?

The composite all species indicator shows the year-to-year fluctuations in population trends across all species, reflecting the observed changes in the annual survey results. Alongside this is the smoothed version of the trend, which is used to formally assess the statistical significance of change over time. The smoothed trend is derived using a published statistical methodology and is used for assessments as it reduces the short-term peaks and troughs resulting from, for example, year-to-year weather and sampling variations. The index is considered to give reliable medium to long-term trends but strong reliance should not be attached to short term changes from one year to the next.

What does a change in the indicator mean?

Key points:

• An increase in the indicator from 100 to 110, for example, would mean an average change of +10% in each of the individual species indices that make up the indicator. This is the same as saying that, on average, there are 10% more individuals in the population of each species.

• A decrease in the indicator from 100 to 90, for example, would mean that the average change in each species index included in the indicator is -10%.

• However, it is important to remember that the all-species index is an aggregate of individual species indices and hence masks a lot of variation among individual species and groups of species. Although its constituent species have shown both massive increases and severe declines, the all-species index has remained largely stable since 1970 as increases and decreases in individual species have tended to balance one another. For this reason, composite indices birds associated with each of the UK’s major habitat types are also calculated (see below). These give a clearer picture of the state of wild birds in each, and help to identify patterns. The best way to see the variation shown by all the different species is to plot all of their trends. Plots for individual species are available on the BTO website.

• If the all species index goes up or down in a given year it does not necessarily mean that the majority of species in the UK have increased or decreased respectively, because the index can be strongly influenced by a large magnitude change in one of the species included (see Example 3 and Example 4, Annex 1) although the effect of a change in one species becomes less influential as the number of species in the index increases.

• Since the all- species index treats every species as equivalent, and does not weight trends by the abundance of each species, a 50% decrease in the population of a common species would have the same effect on the indicator as a 50% decrease in the population of an uncommon species. While the actual decrease in numbers of individuals would be very different, the change in the index is the same.

• The use of equal weights across species ensures that changes in the indicator are not completely dominated by the trends in the most common species. This is apt because long-lived species at the top of the food chain (for example, birds of prey) are naturally much less numerous and numbers tend to fluctuate less than prey species. For example, based on changes in population sizes without the use of equal weightings, declines of 75% in 3 rare species of 100 individuals each, would be completely masked by a 50% increase in one very common species of 10,000 individuals, and lead to a 46% increase in a composite indicator for the 4 species combined rather than the 61% decrease based on equal weightings (see Example 2, Annex 1).

• A conventional average can be overly influenced by one or two extreme values. A geometric mean is an average which is less influenced by any single value far from most others in the data set (an outlier). This is appropriate as outliers in population increases are relatively common. Extreme values can arise from new colonisers, such as little egret, which may increase their population several times over in a short period of time.

Additional indicators

The composite indices for breeding birds in the separate habitat categories are calculated in the same way as described for the all- species index: farmland birds, woodland birds, water and wetland birds and seabirds, as well as for wintering waterbirds. These indices are used an indicators for the state of bird populations within these broad habitat groups. The same species are used in each indicator across years, except where species are added or removed, primarily due to changes in the availability of data and an assessment of its reliability for use in the indicator, following procedures agreed by an Indicators Steering Group and with extensive consultation. A new species can be added without distorting the index by giving it an average value of the index when data for it is not available. Alternatively, if there are earlier data for the new species, the whole index can be recalculated, accepting that this will change the earlier values of the index.

Further information

For more general information on monitoring change in biodiversity using indicators and a discussion of alternative indices, please see.

Buckland, S. T., A. E. Magurran, R. E. Green,and R. M. Fewster. 2005. Monitoring change in biodiversity through composite indices. Philosophical Transactions of the Royal Society of London B 360: 243–254.

For a more detailed discussion of the strengths and weaknesses of using the geometric mean of relative abundance indices to monitor biodiversity, please see:

Buckland, S.T., Studeny, A.C., Magurran, A.E., Illian, J.B. & Newson, S.E. 2011. The geometric mean of relative abundance indices: a biodiversity measure with a difference. Ecosphere 2 (part 9) : 1-19 . View at journal website (DOI: 10.1890/ES11-00186.1)

Annex 1

Example 1

The geometric mean ensures that a doubling in the population index of one species is balanced by a halving in the population index of another species.

Example	Index in year(X)	Index in year(Y)	Change
Species A	100	200	100%
Species B	100	50	-50%
Geometric mean	100	100	0%
Arithmetic mean	100	125	25%

Example 2

The use of equal weights across species ensures that changes in the indicator are not completely dominated by the trends in more common species.

Example	Index in year(X)	Index in year(Y)	Population size year(X)	Population size year(Y)
Species A	100	25	100	25
Species B	100	25	100	25
Species C	100	25	100	25
Species D	100	150	10,000	15,000
Total number of individuals	N/A	N/A	10,300	15,075
Geometric mean	100	39	N/A	N/A
Change in indicator between year(Y) and year(X)		-61%		46%

Example 3

A decline in the index can occur even if more individual species increase than decrease.

Example	Change measure	Population size
Species A	5% increase	105
Species B	5% increase	105
Species C	50% decline	50
Geometric mean		82
Change		-18%

Example 4

An increase in the index can occur even if more individual species decrease than increase.

Example	Change measure	Population size
Species A	5% decline	95
Species B	5% decline	95
Species C	100% increase	200
Geometric mean		122
Change		22%