Countries' agricultural systems have an important impact on biodiversity, for example bird populations. Here, we estimate such impacts by exploiting a natural experiment in the middle of Europe, where there is a naturally homogenous area that is divided into three countries: Switzerland, Germany, and France. These countries have markedly different agricultural systems and policies. Using a methodologically unified and unusually rich bird dataset available across these borders, both for the 2010s and 1990s, we analyze (a) whether there is a clear pattern that bird diversity systematically changes when crossing these borders and (b) whether this has changed over time. To assess bird populations, we focus on Shannon diversity, species richness and number of territories, as well as the individual effect on 29 common bird species. We find that Switzerland has systematically smaller and less diverse bird populations compared to Germany and France, driven entirely by agricultural differences. At the same time, we find that the difference between the countries was considerably more pronounced in the 1990s than in the 2010s. Yet, to reach the bird friendliness of Switzerland's neighboring countries, additional policy effort seems required, for example in the form of targeted agri-environmental payments.

各国的农业系统对生物多样性有重要影响，例如鸟类种群。在这里，我们通过利用欧洲中部的自然实验来估计此类影响，那里有一个自然同质的区域，分为三个国家：瑞士、德国和法国。这些国家的农业制度和政策截然不同。使用 2010 年代和 1990 年代跨越这些边界的方法统一且异常丰富的鸟类数据集，我们分析 (a) 是否存在鸟类多样性在跨越这些边界时发生系统性变化的清晰模式，以及 (b) 这是否已经改变时间。为了评估鸟类种群，我们关注香农多样性、物种丰富度和领土数量，以及对 29 种常见鸟类的个体影响。我们发现，与德国和法国相比，瑞士的鸟类种群系统性较小且种类较少，这完全是由农业差异驱动的。同时，我们发现 1990 年代国家之间的差异比 2010 年代明显得多。然而，要达到瑞士邻国的鸟类友好度，似乎需要额外的政策努力，例如以有针对性的农业环境付款的形式。