Monitoring schemes contributing their data to PECBMS are based on sampling, i.e. population indices and other results are inferred from a sample of sites distributed across a country. The selection of sampling plots (sites) determines how representative the results are.
The most common methods to select sample plots in generic breeding bird monitoring schemes are free choice, systematic selection, stratified random selection and random selection. Definitions according to Sutherland et al. (2004).
Free choice was the common method in older schemes, but nowadays most of these schemes have been replaced with schemes with some element of randomization. Stratified random selection is the prevalent method of newly established monitoring schemes in Europe.
In 9 countries a scheme with free choice was in place by 2008. In four of them, the old schemes with free choice have been replaced by new schemes using stratified random or systematic choice of sampling sites; these new schemes are combined with data from the old schemes. Improvements in scheme design are ongoing in two other countries. In the Netherlands post-stratification and weighting has been used as the method to reduce potential bias (Van Turnhout et al., 2008). Czech Republic coordinators analyzed the main habitats and their coverage by the monitoring, and discovered that only urban habitats are slightly oversampled; important bias is unlikely. Nevertheless, improvements in the sampling design are planned here too.
There are only three schemes where potential bias needs to be addressed better. They will be focus of further efforts to improve sampling design in the near future.
All in all, thanks to the improvements in plot selection and increased rigour that have been applied, we believe that bias which could affect results at the European level is unlikely.
Information on selection of sample plots in national monitoring schemes can be found in Common bird monitoring schemes in Europe.
For more details on sampling strategy see Best Practice Guide(Voříšek et al., 2008) or standard textbooks on monitoring (e.g. Bibby et al., 2000; Sutherland et al., 2004; Sutherland, 2006).