The extraordinary component of PECBMS are the thousands volunteers who count the birds in the field, each year, according to a methodology standardized at national level. The data are sent to the national offices, where, using TRIM software made available by PECBMS (van Strien et al., 2001; Statistics Netherlands, 2017), calculate the national species indices and trends. PECBMS combines national species indices with supra-national indices for individual species for the European Union and its main regions (new and old European Union, and West, South, North, Central and East Europe), plus Europe as a whole. All indices are annually updated, and EU indices are regularly sent to and published by EUROSTAT7. Thirty-nine species compose the Farmland Bird Index (FBI, Table 2). These are common species, which are dependent from agroecosystems for feeding and nesting, and as such are considered to be a descriptor of the state of agroecosystems. The index is a composite, multispecies index calculated using Monte Carlo simulations as described in Soldaat et al. (2017).
Table 2 Species composing the EU farmland bird index
| Alauda arvensis | Emberiza melanocephala | Passer montanus |
| Alectoris rufa | Falco tinnunculus | Perdix perdix |
| Anthus campestris | Galerida cristata | Petronia petronia |
| Anthus pratensis | Galerida theklae | Saxicola rubetra |
| Bubulcus ibis | Hirundo rustica | Saxicola torquatus |
| Burhinus oedicnemus | Lanius collurio | Serinus serinus |
| Calandrella brachydactyla | Lanius minor | Streptopelia turtur |
| Carduelis cannabina | Lanius senator | Sturnus unicolor |
| Ciconia ciconia | Limosa limosa | Sturnus vulgaris |
| Corvus frugilegus | Melanocorypha calandra | Sylvia communis |
| Emberiza cirlus | Miliaria calandra | Tetrax tetrax |
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Emberiza
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