Scientists led by Stanislas Rigal of the Institute of Evolutionary Sciences in Montpellier have found that within 37 years in 28 European countries, the population of farmland bird species has declined by almost 57 percent. In their study, published in the journal PNAS, the scientists summarized observational data from 170 bird species at 20,000 sites over a period from 1980 to 2016. For example, forest birds, migratory birds and species that prefer cold or warm habitats were also examined.

The diversity of the species now studied made it possible to investigate the effects of different stress factors on the animals in different habitats. These included the intensification of agriculture with the use of pesticides and fertilizers, the change in forest area, temperature increases and urbanization. At the same time, bird species found in agricultural areas can provide information about their ecological status. A decline in species, for example, allows conclusions to be drawn about changes in land use and environmental pollution.

The total number of birds of all the species now studied has declined by a quarter during this period. Not only arable and forest-dwelling species are negatively affected by intensive agriculture, but also species that feed on invertebrates, such as the willow (Poecile montanus). Ornithologist Jörg Hoffmann, head of the Sustainable Agriculture and Biodiversity Working Group at the Federal Research Institute for Cultivated Plants in Kleinmachnow, who was not involved in the study, points out that the absence of pesticides leads to a greater diversity of wild plants and thus a greater biodiversity and abundance of insects and birds. According to the study, 143 of the 170 bird species depend on invertebrates as a food source, especially during the breeding season.

Diversity requires diversity

Between 1980 and 2016, the forest area increased in almost all countries, but the number of bird species living predominantly in forests declined by 17 percent, the authors write. Christian Hof, junior research group leader at the Chair of Terrestrial Ecology at the Technical University of Munich, has an explanation for this supposed contradiction: "If these are rather fast-growing monocultures that are supposed to be used for rapid wood production or CO2 storage, this is not necessarily a bird-friendly forest." The latter is more of a diverse, habitat-rich mixed forest with deadwood and bodies of water, which is not actively operated by humans for forestry.

According to the study, the rise in temperature mainly affects long-distance migrants, i.e. birds whose breeding area is very far away from the wintering area – usually several thousand kilometres. Climate change means that food is available earlier and earlier in the European spring. Wintering bird species use these resources in the form of buds and caterpillars and begin breeding, Hof said. Migratory birds such as the black flycatcher (Ficedula hypoleuca), which winters in Africa, then encounter nesting sites that are already occupied and less food.

A decline in birds in Europe also has a direct impact on humans. According to Christian Hof, birds are pest controllers and seed dispersers at the same time. "But they also have an aesthetic value," he says. "Various studies by the Senckenberg Institute have shown that people in areas with more birds are better off."