Effects of nitrogen deposition on territory numbers of breeding birds

Deposition of atmospheric N (nitrogen) is assumed to be a major cause of biodiversity decline in Europe. To date, few studies on the direct or indirect effects of N on bird species have been conducted. Using Swiss bird count data and habitat data, we analyzed the correlation of N deposition with numbers of territories of 112 breeding bird species. Fifty-five species had a negative correlation with N, and 21 had a positive correlation. Thirty-six species showed no clear linear relationship. Insectivorous and herbivorous species were more negatively associated with N deposition (insectivores: 23 species with well-supported negative correlation vs. 9 species with well-supported positive correlation; herbivores: 6 vs. 1) than omnivorous birds or birds feeding on vertebrates (2 with negative correlation vs. 2 with positive correlation and 1 with negative correlation vs. 1 with positive correlation, respectively). Species associated with forest (23 negative vs. 3 positive), human settlement and wetland (each 3 negative vs. 0 positive), and birds that could not be attributed to a single guild (3 negative vs. 1 positive) showed mainly a negative relationship with N deposition, whereas more positive than negative correlations were found for alpine (0 negative vs. 2 positive) and common farmland species (0 negative vs. 7 positive). Ground-nesting species were more negatively associated with N deposition (8 negative vs. 2 positive) than species that nest high aboveground (24 negative vs. 11 positive). The negative correlation of N deposition with territory numbers was slightly more pronounced in long-distance migrant species (9 negative vs. 3 positive) than in resident or short-distance migrants (23 negative vs. 10 positive). Rare species were excluded, likely biasing farmland bird results positively. We assumed that differences in the vegetation due to higher N inputs were the main cause for our results. Reduced plant diversity, altered vegetation structure, and more frequent mowing affect breeding habitat and availability of food (invertebrates and seeds) for birds. In Switzerland, airborne N deposition exceeds by far the critical loads for most ecosystems. Our results highlight the urgent need to reduce N deposition to protect a wide range of Swiss bird species.