A method has been developed to produce supranational yearly totals and their standard errors across countries by combining the national data.
The method considers the differences in population sizes per country and the differences in field methods, and the number 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 produced when the raw data are being used (Van Strien et al., 2001).
To produce supranational indices, the national all-sites totals per species assessed in the national monitoring schemes are combined. A weighting factor is introduced to adjust for differences in national population sizes to ensure 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 (2021) divided by the average of the all-sites totals for referential years defined by the BirdLife International (2021) publication as a year or period of years for which the population sizes are collected. 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, 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 that host at least 50% of the ´PECBMS European´ population of that species. ´PECBMS Europe´ is EU 26 + Norway, Switzerland and the United Kingdom, and consists of those countries which already deliver their data to PECBMS: Andorra, Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Norway, Poland, Portugal, Republic of Ireland, Romania, Slovakia, Slovenia, 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: the Faroe Islands and Greenland, Svalbard, Azores, Madeira, Canary Islands, Gibraltar.
Population species trends (multiplicative slopes) are computed and classified simultaneously at the national level.