National 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 necessarily, 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.
The coordinators of the monitoring schemes produce national species indices. 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. For imputing of missing values is used the predominant statistical technique (Poisson regression) was implemented in the freeware program TRIM (Trends and Indices for Monitoring data, Pannekoek & Van Strien, 2001).
At this moment two types of tools are used to calculate the national index, both using TRIM (available for download here): tool BirdSTATs (available for download) and a code written as three R scripts using RTRIM package, called RTRIM-shell.
The old version of TRIM developed originally in Delphi has been outdated for some time and couldn´t be updated anymore. We therefore strongly recommend using the RTRIM-shell. PECBMS plans to leave the TRIM and Birdstats in the near future and to use only RTRIM-shell for the national index calculation.
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 the 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 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 Time totals for each species. This data is collected by the PECBMS coordinator together with their standard errors and covariances.
For details on TRIM and BirdSTATs, see also Box Missing values I.
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), we need to combine the results per scheme to produce new combined indices per species and country. The procedure resembles the one to produce supranational indices from national results (see detail in 2.2. Combining national data into supranational outputs). This combination is integrated into the calculation of supranational indices and all are performed at once in one tool.
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).