Population yearly indices and trends are the most important outputs of national monitoring schemes. The index gives bird numbers in percentages relative to a base year, when the index value is set at 100%. Usually, but not necessary, the first year of a time series is chosen as the base year. Trend values express the overall population change over a period of years.
National species indices are produced by the coordinators of the monitoring schemes. They assess yearly all-sites totals per species and compute the individual national species indices in a prescribed way. The count data usually contain missing values, and to impute these they use the predominant statistical technique, that is, Poisson regression, as implemented in the TRIM software (Trends and Indices for Monitoring data, Pannekoek & Van Strien, 2001). TRIM is a widely used freeware program (available for download here). To facilitate the use of TRIM, the software tool BirdSTATs is available for download, too.
Statistically spoken: the basic TRIM model contains both site effects and year effects and estimates missing values from the data of all surveyed sites:
ln μij = αi + γj,
with αi the effect for site i and γj the effect for year j on the natural log of expected counts μij. Missing counts for particular sites are estimated (´imputed´) from changes in all other sites, or in sites with the same characteristics if the basic model is extended with covariates. The assumption is that changes observed in surveyed sites also apply to non-surveyed sites.
The program produces imputed yearly indices and totals for each species. These yearly scheme totals, together with their standard errors and covariances, are collected by the PECBMS coordinator.
In case there is more than one monitoring scheme within a country, e.g. an old scheme and a new one (i.e. schemes differ in time span) or different regional schemes (i.e. schemes differ in spatial coverage), the coordinator combines the results per scheme to produce new combined indices per species and per country. A tailor-made software tool called Combine has been developed for this purpose, which also takes into account standard errors of indices of the constituent schemes. The procedure used resembles the one to produce supranational indices from national results (see below).
In addition to national indices, trends are computed to indicate whether long term changes in bird populations are strongly increasing, moderately increasing, stable, uncertain, moderately declining or steep declining (learn more in Box Trend interpretation and classification).