How feeding birds may be polluting local environments

New research led by an NAU alumnus shows that backyard bird feeders, although put out with the best of intentions, is changing the chemistry of local ecosystems, including introducing a potentially harmful amount of phosphorus into the environment.

The study, published Aug. 7 in Frontiers in Ecology and the Environment, shows that, collectively, the millions of household bird feeders throughout the world add up to a large transfer of additional nutrients into local environments. This artificial supplementing of a natural process may lead to unanticipated ecological impacts.

“The seeds, nuts and grains provided for birds are rich with fats and proteins,” said NAU alumnus Andrew Abraham, a researcher from Aarhus University in Denmark and lead author of the study. “Yet most of this nutrient-rich bird food is sourced on the global market and transported thousands of kilometers to feeder stations.”

“Birds are incredibly effective dispersers,” said Ethan Duvall, a researcher from Cornell and co-author on the study. “They eat a lot, fly a lot and poop a lot. This means that extra nutrients provided at feeders eventually make their way into local ecosystems.”

In their research, which examined the United Kingdom as a case study, Abraham and colleagues found that the scale of phosphorus supply in supplementary foods provided for garden and game birds equalled about 2.4 Gg/year.

While phosphorus is an essential element for both plants and animals, too much of it can cause problems. Concerningly, phosphorus release by bird feeding was comparable to other human activities, such as industrial waste and sewage misconnects, which are considered environmental polluters.

The greatest supply of extra nutrients was provided for game birds. The researchers found that each year, more than twice as much phosphorus was provided to pheasants, partridges and other game birds compared to garden birds.

This supplementary food constitutes a significant component of game bird nutritional intake and helps elevate densities beyond the natural carrying capacity. While this may benefit land owners in the short-term, it could spell ecological trouble in the long-term.

“When additional nutrients from bird feeding enter natural environments, they have profound consequences for the species that live there,” Abraham said. “If bird feeding occurs at high rates or over long periods of time, it can begin to change the fertility of the landscape, upsetting the balance of the ecosystem.

“Eventually, the nutrients in bird feed are washed into rivers, where they may also exacerbate issues of eutrophication”—that is, they may produce too many rich nutrients, causing algae to grow too densely and harming animals that live in or near the rivers.

Does this mean bird feeders are bad? It’s not that simple, the researchers say. The bigger question is, how do different factors interact to change the environment.

“It is important to study animal dispersal of nutrients, because in some places, animals can spread important nutrients in a way that benefits ecosystems, but in other cases, like in the case of bird feeding, they may spread too many nutrients, causing problems,” said Chris Doughty, co-author and professor of ecology at Northern Arizona University.

“We must continually re-evaluate bird feeding practices as new evidence arises,” said Kate Plummer, senior research ecologist at the British Trust for Ornithology. “Previous research has already shown that bird feeding is linked to changes in bird community composition, behavior, distribution and transfer of avian disease.

“While our study suggests that the impact of garden feeding is significantly less than that of gamebird feeding when it comes to nutrient dispersal, the findings add a new dimension to our efforts to understand the balance of costs and benefits of putting out food in our gardens.”