Airborne Nitrogen Deposition Affects Bird Populations

Nitrogen from agriculture, traffic, and industry enters soils and water bodies via the air. A new study shows for the first time that increased nitrogen deposition has a negative impact on bird populations – even in semi-natural habitats.

Nitrogen from agriculture (65%), traffic (22%), and industry (10%) enters the atmosphere as ammonia and nitrogen oxides and can be transported over long distances before it deposits into soils and water bodies. This input into habitats such as forests, meadows, pastures, lakes, bogs, and wetlands is considered as one of the main drivers of biodiversity loss in Europe.

Until now, there have been few studies on the effects of nitrogen deposition on bird populations. Using data from the current Swiss Breeding Bird Atlas 2013–2016 and nitrogen deposition data from the Swiss Federal Office for the Environment (FOEN), we investigated the relationship between nitrogen deposition and territory density of 112 common breeding bird species in Switzerland.

Negative correlation between territory density and nitrogen deposition in many species

Fifty-five species showed a negative relationship with increasing nitrogen deposition. For 21 species a positive correlation was found, and 36 species showed no linear relationship. Particularly negatively affected were insectivores and seed-eaters, forest and wetland species, ground-nesting birds, and long-distance migrants. Among the 21 species with a positive correlation were several farmland species: These are common birds in Switzerland such as the Red Kite, White Wagtail, and Starling, which seem to cope with current nitrogen levels. Many of today’s rare farmland species, such as Corn Crake, Lapwing, Meadow Pipit, or Corn Bunting, require extensively used habitats (also for nesting) and are therefore expected to show a negative correlation with nitrogen deposition. However, due to their rarity, they could not be included in the analysis.

The effects of nitrogen on birds are likely due to changes in vegetation. Nitrogen deposition in nutrient-poor sites and ecosystems causes a fertilization effect, resulting in denser and more uniform vegetation, which makes foraging and nesting more difficult for some species. Additionally, vegetation in formerly nutrient-poor meadows grows faster and thicker, leading to earlier and more frequent mowing. This results in significant losses of insects, bird nests, and sometimes even adult breeding birds. All these changes also impair insect diversity, an important food source for many bird species.

Semi-natural habitats are also affected

The study shows: High nitrogen deposition from agriculture, industry, and traffic negatively affects many bird species – not only at the site of nitrogen emission but also in semi-natural habitats like forests and wetlands. In most ecosystems in Switzerland, airborne nitrogen deposition significantly exceeds the critical loads, beyond which biodiversity is impaired. Our findings highlight the urgent need for action. Current efforts to reduce nitrogen emissions show some success but fall far short of what is needed to effectively halt biodiversity loss.