2. Supranational species indices and trends


2.1. Delivery of national data to PECBMS coordination unit
Box Data checks
Box Participating countries
2.2. Combining national data into supranational outputs
Box Missing values II
2.3. Types of supranational results that PECBMS produces

2.1. Delivery of national data to PECBMS coordination unit

Coordinators of national monitoring schemes deliver their national data to the PECBMS coordination unit annually. For information on data and contributing countries see Box Participating countries.

The data delivered are: the national yearly indices per species, the all-sites yearly totals (= the sum of birds counted across all sites per year) and their standard errors, and the covariances between the yearly figures.
Specifically, national coordinators deliver two files per species - so called out and ocv files (see paper by Pannekoek & Van Strien, 2001 or also see website of Statistics Netherlands) - both produced by TRIM when calculating species indices at the national level. These TRIM output files are accompanied by a species list and by comments indicating any potential problems in the data. These comments are taken into account in further data analysis.
National data are checked for their quality using quantitative criteria (see Box Data checks).

Box Data checks

After receipt, the PECBMS coordination unit checks the national data on species numbers, indices and trends for inconsistencies; it checks for instance if the scheme all-sites totals exceed the national population size known from other sources. Suspicious data coming from countries with more than 10% of PEBCMS European bird populations (see below) are examined in detail. If any problems are encountered, the coordinators of national monitoring schemes are consulted. Additionally, documentation from previous data analysis is checked and used for decisions in some cases. Data with persistent serious data problems are excluded from computation.

Suspicious data - national species data are subject to closer examination when:
  • Slope (Multiplicative) < 0.6
  • Slope (Multiplicative) > 1.5
  • Slope (Multiplicative) standard errors > 0.5
  • Percentage of scheme time totals of the species > 95% of national population size of the species in Birds in Europe 2 (BirdLife International 2004)
  • Ratio of national population size to scheme time totals > maximum of species population size in Birds in Europe 2 (BirdLife International 2004)
  • Number of zero counts < 1
  • Number of missing counts < 1
  • Index value < 0.5
  • Index value > 1000
  • Scheme time totals < 1
  • Scheme time totals > 1000000
  • More than one year with index = 100 and SE = 0 present in the results

For these data checks an automatic system has been developed by Statistics Netherlands.

All national indices - species by species - are also checked for their interannual consistency (comparison with previous trends and indices) and all suspicious and inexplicable inconsistencies in indices are examined in detail. Also for this procedure a special tool has been developed.

Box Participating countries

Monitoring data available to PECBMS come from more than 20 countries. The number of countries contributing data and the number of species covered have increased almost every year, as new schemes have started up and provided their data. Almost all EU Member States are now represented, along with some neighbouring countries.

In 2011 data came from 25 countries: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom.

Number of countries contributing to the PECBMS has increased since 2003 from 18 to 25. Moreover, some of the countries with a scheme established recently have not contributed their data to PECBMS yet, but are expected to do this in near future. The overall number of species for that the PECBMS produces trends and indices has almost tripled since 2003 (see graph below).

For details and updated information on national monitoring schemes browse country reports, for list of countries contributing to earlier versions of European trends and indicators see Archive.

2.2. Combining national data into supranational

A method has been developed to produce supranational yearly totals and their standard errors across countries by combining the national data. The method takes into account the differences in population sizes per country, as well as the differences in field methods and in the numbers of sites and years covered by the national schemes. Instead of deriving the standard errors in the usual statistical way from count data and model fit, standard errors (and the year-year covariances) that resulted from the calculation of the all-sites totals per country were applied. The results are similar to those that would be produced when the raw data are being used (Van Strien et al, 2001).

To produce supranational indices, the national all-sites totals per species as assessed in the national monitoring schemes are combined. A weighting factor is introduced to adjust for differences in national population sizes, to make sure that a change in a larger national population has an accordingly greater impact on the overall trend than a change in a smaller population. The weighting factor is calculated as the national population size derived from BirdLife International (2004) in 1999-2000 divided by the average of the all-sites totals for 1999-2001. This weighting factor is applied to all other years of the scheme. By this weighting, the yearly scheme totals are converted into yearly national population sizes.

The national European monitoring schemes have started in different years, leading to missing national all-sites totals. An adapted version of TRIM is used to estimate the missing country totals, in a way equivalent to imputing missing counts for particular sites within a country. After these weightings and imputation steps, the national totals are summed up to European totals.

European species indices for a species are computed if data are available from countries which together host at least 50% of the PECBMS European population of that species. PECBMS Europe is EU 27 + Norway and Switzerland and consists of those countries which already deliver their data to PECBMS or are supposed to do so in the near future: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, Netherlands, Norway, Poland, Portugal, Republic of Ireland, Romania, Slovakia, Spain, Sweden, Switzerland, United Kingdom.

However, some parts of these countries (states) were excluded, mainly because of their far distance to the mainland of Europe: Faroe Islands and Greenland, Svalbard, Azores, Madeira, Canary Islands, Gibraltar.

Population species trends (multiplicative slopes) are computed and classified in the same way as at the national level.

Box Missing values II

Ideally, in a European breeding bird monitoring scheme all countries start in the same year. If so, it is relatively easy to assess the changes in the yearly all-countries totals of breeding pairs.

The procedure then is as follows. First the national yearly all-sites totals are converted into national yearly total population sizes in that country. To do so, information of total population sizes collected by BirdLife International (2004) is used. A weighting factor is calculated by dividing the population total as assessed by BirdLife International (2004) by the estimated all-sites totals for those years that are covered by population size estimates (BirdLife International 2004) and a monitoring scheme. Subsequently, this weighting is applied to all years of the monitoring scheme, so that the weighted year totals may be considered as the yearly population totals in that country. An example may clarify this. Suppose the estimated all-sites total of species X in the UK amounts 100 in the year 2000 and 110 in 2001. If the population total for the UK would be 1000 in the year 2000, the weighting factor equals 1000/100 = 10. For the year 2001, the population total is 10*110 = 1100.

The standard error of the yearly population totals are (weight factor) x (standard error of all-sites total).

Subsequently, the population totals for each country are summed to yield the supranational population totals for each year. The standard errors of the supranational totals were derived by the following statistical rule: variance of supranational total = variance of country1 total + variance of country2 total + variance of country3 total etc., where variance = standard error2. This rule applies because the estimates of the yearly totals are independent between countries. Finally, the supranational totals are converted into indices.

The national European monitoring schemes, however, started in different years, leading to missing national all-sites totals. Just as was explained in the example for sites above, simple comparisons of the yearly sum of country totals will then give misleading inferences on trends, because lesser countries contribute in earlier years. Again we used an adapted version of TRIM to estimate the missing country totals, in a way equivalent to imputing missing counts for particular sites. Basically we computed indices for four European regions (West Europe, North Europe, Central & East Europe and South Europe) and derived missing national totals from changes in countries in the same European region, as the example shows.

year 1
year 2
year 3
Country 1 population total
Country 2 population total
Country 3 population total
West Europe population total

Thereafter, the regional totals were calculated and summed to obtain European indices as described above.

An extra complication is that also regions differ in the years covered. For instance, the first country in South Europe started its monitoring activities only in 1989, whereas monitoring started in 1982 in Central & East Europe and even earlier in West Europe. To combine totals from the different regions, we performed a third step to combine results using TRIM. All-together we applied a refined hierarchical imputation procedure to combine country population totals.

To produce supranational indices, we used a slightly adapted version of TRIM tailored to combining all-sites totals and their standard errors instead of raw counts per site. Instead of deriving the standard errors in the usual statistical way from count data and model fit, we applied the standard errors (and the year-year covariances) that resulted from the calculation of the all-sites totals per country.

In summary, PECBMS combines data from national monitoring schemes which differ in several aspects. These differences are addressed in order to produce unbiased results with known precision (Van Strien et al., 2001).

Overview of effects of methodological differences in national monitoring schemes:

Difference between countries (national schemes)
Influence on national indices
Consequences for supranational indices
Field method
Include standard error
Number of sites
Include standard error
Site selection method
Remove bias at national level
Index method
Use proper method (TRIM)
Years covered
Missing yearly indices
Estimate missing indices
Population size
Weight indices by national population size

When supranational indices and trends are produced, a control for interannual consistency is carried out just as was done for national data (see details in chapter 4., Quality control). In case some inconsistencies had occurred, they are examined in details to seek whether the inconsistency is caused by enlarged or improved data set or by computation errors.

For details about imputing countries, see computation schedule.

2.3. Types of supranational results that PECBMS produces

Species indices and trends are produced for Europe and its regions (Central & East, North, South, West, and Southeast Europe, East Mediterranean and West Balkan) and for EU and its regions: New (since 2004) and Old EU Members States).

The species indices are always computed for maximum time period given by the country with the longest data set within the region (e.g. indices for West Europe are produced since 1966 given by the starting year of the indices from UK, while indices from South Europe are produced since 1989 given by the starting year of indices from France - see an overview below).

Overview showing list of 25 countries, their start years and region that each country belongs to:

Country/regionRegion(group of countries)First yearLast year
Cyprus2) East Mediterranean20062009
Czech RepublicCEE19822009
Germany East4)CEE19912009
Germany West4)WE19892009
Republic of IrelandWE19982009
SloveniaWest Balkan20072009
United KingdomWE19662009

WE - West Europe
NE - North Europe
SE - South Europe
SEE - Southeast Europe
CEE - Central & East Europe
First year - first year of data time series in a country/region
Last year - last year of data time series in a country/region
Time series for individual species from national schemes are shorter in certain cases.

Countries notes:
1) Data for Belgium were combined from Wallonia and Brussels regions.
2) Data for Cyprus come from two schemes that partly differ in their regional coverage, Volunteer Common Birds Census (2006-2009) and Western Cyprus Common Bird Census (2007-2009). Data from both schemes were combined.
3) Data for France come from two schemes, old (1989-2001) and new one (2001-2009). Data from both schemes were combined.
4) Data for Germany were combined from schemes in former East and West Germany.
5) Data for Latvia come from three different schemes, two old ones (differ in their regional coverage, and cover the periods 1995-2006 and 2003-2006, respectively) and a new one (2005-2009). Data from all schemes were combined.
6) Data for Norway come from two schemes, Norsk Hekkefugltaksering, HFT (1996-2009), and Terrestrisk overvåking, TOV-I (1996-2009). Data from both schemes were combined.
7) Data for Spain come from two schemes that differ in their regional coverage, Seguimiento de Aves Comunes Reproductoras en Espańa, SACRE (1998-2009), and Seguiment d'Ocells Comuns a Catalunya, SOCC (2002-2009). Data from both schemes were combined.
8) Data for Sweden come from two schemes, old (1975-2009) and new one (1998-2009). Data from both schemes were combined.

However, we publish the European indices since the year 1980 at the earliest since when the index is based on data from several countries.

The species trends are produced for the maximum time period and for shorter periods as well: since 1980, 1990, 1995, 2000, and 2005. For simplicity only, the trends since 1980 and 1990 are published.

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