Birds play an outsized, underappreciated role in shaping Earth’s chemistry, distributing nutrients across oceans, forests and coral reefs and sustaining life far beyond the nest.
Researchers from Cornell and five other institutions reviewed nearly 400 scientific papers to show how birds transport nutrients from land, lake and sea through their waste and bodies. This influences everything from soil and water chemistry to plant and animal growth. The work published Oct. 19 in Current Biology.
“You can think of animals – and especially birds – as highly efficient and important nutrient transporters or nutrient processing machines, concentrating nutrients in their waste and then depositing them into ecosystems that become valuable resources for all sorts of plants and living organisms,” said lead author Ethan Duvall, Ph.D. ’26, who earned his doctorate in the graduate field of ecology and evolutionary biology.
That’s because flight allows birds to traverse vast distances and barriers such as oceans and mountains. Birds are also extremely efficient at processing and recycling nutrients. Their food consumption rates, according to Duvall, are about 30% higher than similar sized mammals. As a result, birds are constantly eating – feeding on seeds, insects, invertebrates and fruits that are rich in nitrogen and phosphorus – and releasing nutrients at elevated rates.
Birds that congregate at nesting or roosting sites, such as seabirds, deposit high concentrations of nutrients in small areas, providing plants and other organisms with important resources. “It can drive higher amounts of plant and algae growth, but it can also help consumers of those plants, like insects and other vertebrates, maintain a more nutrient-rich diet. So these islands of fertility can have cascading effects,” Duvall said.
Nutrients from birds, and in particular seabirds, also wash into nearby coastal waters, acting as nutrient lifelines for coral reefs and the fish communities that depend on these habitats, according to the studies surveyed. Where seabird colonies have disappeared, nutrient levels decline, resulting in reduced plant and algae productivity, reef fish biomass and coral reefs’ resilience to environmental stress.
Conversely, bird waste can help restore plant communities. “We see a lot of potential for harnessing birds’ capacity to transport recycled nutrients to aid restoration,” said Duvall. One study found that installing seabird perches within seagrass restoration sites attracted birds that deposited large amounts of phosphorus, helping the seagrass recover.
“While ecologists have long known that birds transport nutrients, our review establishes the magnitude and geographic scale of that role,” said study co-author Amanda Rodewald, Garvin Professor and senior director of Center for Avian Population Studies at the Lab of Ornithology, and a professor of natural resources and the environment in the College of Agriculture and Life Sciences (CALS).
Preliminary findings of the survey research prompted Duvall to take on a different study. Whereas most of the papers in the survey research focused on a single species, Duvall wanted to measure the contributions from different types of birds in one area that would help them quantify nutrient release from communities of birds.
So he, Linsey Chen ’24 and Alexander Flecker, professor of ecology and evolutionary biology (CALS), analyzed bird waste within a songbird community in spring 2021 and fall 2022 at New York’s Braddock Bay Bird Observatory on Lake Ontario.
The team found that seasonal shifts in the birds’ diet created major changes in the composition of their waste. In the spring, when birds eat more insects, their waste tends to be richer in nitrogen. In the fall, when birds consume more fruits and seeds, their waste becomes richer in carbon.
The findings from this second study, published in March in Functional Ecology, show that differences in body size, behavior, diet and seasonal differences in diets all affect the quantity of nutrients birds release. Scientists can then use this information to predict how declining bird populations could cascade to broader ecological change, Duvall said.
With 45% of global migratory bird species declining, according to the State of the World’s Birds report, many ecosystems may become less fertile, and therefore less productive. These changes can also affect ecosystems’ ability to sequester carbon, Duvall said.
“You actually see these really strong, cascading ecological responses when you sever that consistent deposit of nutrients that birds supply,” he said.
Kathi Borgmann is communications manager at the Cornell Lab of Ornithology.
