Counting birds all year round

At the Swiss Ornithological Institute, one of our core tasks is to keep track of the presence and numbers of breeding and wintering birds in Switzerland. We have several tools to help us do this, most importantly our monitoring programmes.

Late snowmelt, rockfall, sudden changes in weather and long distances make surveys in the alpine zone extremely challenging.
Photo © Roman Graf

The areas in the scheme MHB have been surveyed annually since 1999, those in the BDM scheme are visited every five years.
Photo © figure: Swiss Ornithological Institute; map: swisstopo

Thanks to many survey sites in the Alps, we now have reliable trends for species that Switzerland has international responsibility for, like the Citril Finch. The index shows the relative trend of the breeding population in Switzerland (100 = mean 1990– 2021).
Photo © figure: Swiss Ornithological Institute; photo: Ralph Martin

The Swiss Ornithological Institute favours a simplified territory mapping method. Observers benefit from a range of tools to support them: an app facilitates surveys in the field, Terrimap online supports the digitisation and processing of the records, and Autoterri is a new tool for automatic territory delimitation. The example shows records of Eurasian Blackbird.
Photo © figure: Swiss Ornithological Institute; map: swisstopo

The SBI ® provides population trends for individual breeding bird species, but also for species groups. The graph shows population trends for the species on the Red List. The pie chart segments indicate the percentage of breeding bird species that have disappeared (black), are declining (red), fluctuating (blue), increasing (green), or have newly occurred (grey).
Photo © figure: Swiss Ornithological Institute

Because they have wings to fly, birds are much more mobile than other groups of animals. Keeping track of these movements keeps the coordination team in Sempach busy all year round. Thanks to the approximately 5,000 people active on ornitho.ch, standardised waterbird counts on more than 300 sections of rivers and lakes, and ringing stations in wetlands and on mountain passes, the presence of wintering and migrating birds is well documented. Monitoring the species that regularly breed here – around 180 – takes more time and effort. Numbers are currently available for 176 species, in most cases starting in 1990. At the heart of our monitoring efforts is the common breeding bird monitoring scheme or MHB (in German, Monitoring Häufige Brutvögel): since 1990, it has recorded the population sizes of common and widespread breeding birds, including many passerines, in 267 kilometre squares across Switzerland. Additional data come from the national biodiversity monitoring programme (BDM), which surveys about 500 kilometre squares every five years. On top of this, the Swiss Ornithological Institute collaborates with local partners, ornithological groups and species experts to conduct surveys in approximately 100 wetlands, special habitats, military training grounds and parks. And finally, we organise counts of colonial species as well as special surveys, for example of nocturnal species or cliff-nesting birds.

Territory mapping: a proven method

Many surveys use a simplified form of territory mapping, which involves recording the location of all birds detected in a kilometre square while following a predetermined route. The method has a long tradition in Switzerland; it is well-suited to the often rather small habitats and the results are easy to communicate. The quantitative surveys conducted for the breeding bird atlases 1993–1996 and 2013–2016 used the same method. As a rule, three survey visits are undertaken in every area, although some Alpine areas are only visited twice, while wetlands and special habitats are surveyed in five to six visits.

Various tools help us to keep the territory mapping method as simple and as standardised as possible: Observers receive detailed maps that indicate the routes and the duration for the survey visits, and migratory species that arrive late in the season are only counted after a given date. A mapping app developed by the Association of German Avifaunists (Dachverband Deutscher Avifaunisten DDA), a digitisation tool (Terrimap online) and software for automatic territory delimitation (Autoterri) are available to us today to help collect and analyse data efficiently. The goal is always to make any desk work as easy as possible for the surveyors.

Mountain birds are especially challenging

The Alps cover 58 % of the territory of Switzerland, the Jura mountains Jura 11 %. Because our country has a special responsibility for the conservation of birds in alpine and subalpine habitats, it is important that these areas are well represented. We survey areas up to 2,500 m above sea level. Surveys in these areas are particularly challenging: weather changes, late snowmelt, rockfall, and bridges or paths washed away by avalanches or floods can cause problems. Moreover, the Alps are home to many species that are extremely hard to survey. Birds like Rock Ptarmigan, Rock Partridge, Rufous-tailed Rock-thrush and Wallcreeper are well camouflaged or use vast habitats in rugged terrain where the view is often obstructed. The Alpine Accentor moves in family groups, the Yellow-billed Chough forages in flocks and covers great distances, as do White-winged Snowfinch, Redpoll and Common Linnet. Fortunately, other species like Water Pipit and Northern Wheatear are more territorial and easier to record.

Combining data sources for more accurate trends

In a small country like Switzerland, it is particularly difficult to collect enough data to calculate meaningful trends for scarce and elusive species. Besides mountain birds, these include grouse, birds of prey, woodpeckers and some rare songbirds. The general monitoring programmes do not record them frequently enough for us to calculate trends. Statistical methods developed at the Swiss Ornithological Institute now allow us to combine data from different sources for trend calculation. We combine opportunistic observations from ornitho.ch with the quantitative data collected for the MHB, BDM and wetland schemes as well as for the breeding bird atlases. Complementing opportunistic observations with more standardised data is valuable for two reasons: Firstly, the monitoring schemes survey the sample areas regularly following rules that stay the same over time, which facilitates the assessment of long-term trends. And secondly, we can incorporate changes in population density per kilometre square – information that cannot be drawn from opportunistic observations. The resulting trends differ quite substantially from those based solely on opportunistic observations, especially for the 1990s. An in-depth analysis has confirmed that the new method produces more reliable trends in many cases.

Treasure trove for methodological research

Over the years, the standardised MHB surveys have proven to be an excellent source of raw material for novel statistical analyses. In particular, the longterm collaboration between the Swiss Ornithological Institute and Andy Royle from Patuxent Wildlife Research Center (USA) has resulted in new analytical methods that take into account the probability of detecting a species. The new methods provide insight into presence and population size, changes in these variables, as well as environmental factors that influence them. As a consequence, the MHB scheme has become known worldwide and figures prominently in numerous research articles and even textbooks. Its value thus goes far beyond measuring the natural diversity of birds in Switzerland. MHB is a true treasure trove of data that serves to develop new methods and verify fundamental biological hypotheses.

Growing use of data

Looking back at the past decades, it is clear that the data produced by the Swiss Ornithological Institute’s monitoring projects are in growing demand. Requests range from simple database extracts to assess infrastructure or restoration projects, to scientific analyses and modelling, all the way to large data packages for the EuroBirdPortal or other international projects. Furthermore, our methodological innovations and the outputs from our basic research are in greater demand than ever. But none of this would be possible without the engagement of countless motivated volunteers. Every project begins with their willingness to go out and collect the best possible data even in extremely challenging terrain. A heartfelt thank you to all of them!