© Marcel Burkhardt
Since 1999, the populations of about 150 breeding bird species are surveyed in 267 1-km squares laid out as a representative grid across all of Switzerland. Annual population trend estimates are produced for about 70 of the more common and widespread species.
Our common breeding bird survey ("Monitoring Häufige Brutvögel" MHB) is pivotal for monitoring the populations of the more common species in terms of trends and changes of range size.
The MHB sample consists of 267 1-km squares that are laid out as a grid across Switzerland. Most of the MHB sample squares form a subset of the larger sample (ca. 500 squares) surveyed as part of the federal programme "Biodiversity Monitoring Switzerland", with which we have a close collaboration. Fieldwork is conducted by about 200 skilled birdwatchers, most of them volunteers, whose collaboration is invaluable for this project. Avian populations are monitored using a simplified territory mapping protocol, where each square is surveyed three times and above the timberline twice, respectively, during the breeding season. Surveys are conducted along a square specific transect route that does not change over the years. For each detected species, only records that meet certain conditions (e.g. in terms of survey date) are retained.
Owing to a standardised field protocol and an intense search effort (a typical transect route being 4–6 km and each survey lasting about 3–4 h), the MHB produces precise estimates of population trends for most common species.
As in all bird monitoring programmes, imperfect detectability is an important challenge for the interpretation of MHB data. That is, hardly ever can all occupied territories, occupied squares or occurring species be found. If there is an interest in absolute, not relative, numbers, or if the proportion that is found changes over time and thus introduces spurious patterns into the observed counts, then absolute population size or species richness must be estimated via an estimate of detectability. Over the last few years, MHB data have been instrumental to the development of new statistical methodology, which is useful for the estimation of abundance, occurrence and species richness in large-scale monitoring programmes.
Moreover, MHB data, like those from other modern, national monitoring programmes, have an unsurpassed spatial and temporal extent that will make them invaluable for answering basic scientific questions. For instance, data from atlas or monitoring projects are increasingly being used to explore the response of bird communities to climate change. It is likely that MHB data will continue to make important contributions to this and also to other areas of biological research.
Hence, above and beyond their basic function to inform us about population trends of Swiss birds, MHB data represent an important asset for the development of new methods of data interpretation as well as for tests of basic scientific hypotheses.