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Birds swarm around a tractor

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Populations have been declining for decades: between 1980 and 2016, the number of birds in Europe fell by around a quarter. The authors of a recent study have identified intensive agriculture as the main cause. Other reasons for the decline are urbanization and the rise in temperature, they say. It is interesting to note that some bird species can benefit from the changes – but the minority.

A team led by Stanislas Rigal and Vincent Devictor from the Institut des Sciences de l'Évolution de Montpellier reports on the results in the journal "Proceedings of the National Academy of Sciences". The team had evaluated observational data on 170 common bird species at more than 20,000 sites in 28 European countries. They also used official statistics, such as those from the European Union Statistical Office (Eurostat) and the Food and Agriculture Organization of the United Nations (FAO).

The decline in bird populations is therefore unevenly distributed. Birds that prefer farmland as a habitat were particularly affected, with a reduction of almost 57 percent. In the case of birds in cooler habitats, populations declined by 40 percent. Bird populations that inhabit warmer habitats and are mainly found in forests shrank less sharply, by a good 17 percent.

With the help of statistical trend analyses, the scientists determined the magnitude of various factors influencing bird populations. According to the report, intensive agriculture, which uses pesticides and fertilizers, is the main cause. Pesticides reduced the number of insects, which in turn serve as food for many bird species, the study says. This explains why those bird species that prefer arable land as a habitat are particularly affected. Of the 55 forest dwellers analyzed, on the other hand, about half showed a negative trend, the other half a positive trend.

Call for a change in agricultural policy

The current results suggest, the researchers write, "that the fate of common European bird populations depends on the rapid implementation of transformative changes in European societies and, in particular, on agrarian reforms."

The results of the study are not surprising for Christian Hof of the Technical University of Munich. He was not involved in the research work. "The new achievement of the study is the large scale, the data quality and the explicit linking of bird data with the intensity of agriculture and other factors," he concludes in an interview with the Science Media Center.

However, Jörg Hoffmann of the Julius Kühn Institute in Kleinmachnow, who was also not involved in the study, also criticizes the authors' failures. For example, the periods and methods of bird surveys have not been aligned. "With regard to climate change, for example, in addition to air temperature, precipitation, its seasonal distribution and changes in the landscape water balance would also have to be taken into account," says the researcher.