1.1 Counting birds
Birds are counted using standardised field methods (for details see Box Field methods). Since complete counts are nearly impossible for large spatial units, birds are counted at sample plots selected across a territory of a country (for details see Box Selection of sample plots). Although field methods, selection of sample plots, and also number of years covered differ among European countries, statistical methods can handle this (see chapter 2.2. Combining national data into supranational outputs). Birds are counted within national generic breeding bird monitoring schemes, where all species registered are counted. However, some species are not covered well by these schemes, such as species with nocturnal activity (e.g. owls) or cryptic life style, some clustered and colonial species or extremely rare species. For such species or groups of species, specific surveys need to be set up, but these are not the focus of PECBMS.
Survey results can be affected by the fact that only part of the birds present at a particular site at the moment of counting is detected by an observer. This ´detection probability´ is variable over space and time and may also differ between observers. This should be addressed in field methods and data analysis. New methods are currently being developed to do so.
The majority of field work (i.e. bird counts) is done by volunteers and managed by coordinators. Since bird watching is a widespread activity across Europe and elsewhere, it is often no problem to recruit a high number of volunteers for bird surveys, and this is relatively easy in comparison to other taxa. Just as professionals, volunteers must be able to identify the birds in the field properly, record field data accurately and in proper format and deliver them timely to the coordinators. Since the volunteers do their work for free, one might fear that their work suffers from this, but such fear is unnecessary. Coordinators use a wide array of methods to check the skills of the volunteers and to guarantee a high standard of the data delivered.
One possible problem connected to working with volunteers is the selection of sampling plots. Volunteers might prefer to count in areas that are rich in birds rather than to be directed to plots which have been selected randomly. To solve this problem several national monitoring schemes select sample plots in a stratified random manner. Another problem is that volunteer fieldworkers can leave a scheme at any moment, causing a turnover in the sites counted and missing values. This occurs in any long-term monitoring scheme and statistical techniques and software are widely available and used to solve this problem too.
While the potential risks linked to the involvement of volunteer fieldworkers have been solved, several advantages remain: the running costs of a scheme are relatively low and large-scale schemes are feasible (Greenwood, 2007). If you are eager to see actual methodical article about PECBMS work read Gregory et al., 2019.
For more details on each national monitoring scheme visit Common bird monitoring schemes in Europe.
Box Field Methods