Birds do more than carry seeds: Their movement and waste recycle nutrients in ecosystems ranging from ocean islands and coral reefs to a migratory stopover on Lake Ontario, according to Cornell-led research.
Researchers from Cornell and five other institutions reviewed nearly 400 scientific papers for a study published in Current Biology on Oct. 19. The review brings together evidence of how birds move nitrogen, phosphorus and other materials across land and water, according to the Cornell Chronicle.
Seabird colonies concentrate nutrients
Flight allows birds to cross oceans, mountains and other barriers while carrying nutrients in their bodies and releasing them through waste. Lead author Ethan Duvall, who earned his doctorate at Cornell, said birds' food consumption rates are about 30% higher than those of similarly sized mammals, increasing the amount of material they process and release.
The effects can be especially concentrated where birds gather to nest or roost. Seabirds, for example, bring nutrients from the sea onto land. Their waste can support plant and algae growth, with effects that extend to insects and other animals feeding on those plants, Duvall said.
Nutrients from seabird colonies can also wash into coastal waters. The studies reviewed link those inputs to coral reefs and reef fish communities; in places where colonies have disappeared, researchers have observed lower nutrient levels, reduced plant and algae production, less reef fish biomass and weaker coral resilience to environmental stress.
One study cited by the Cornell team tested seabird perches at a seagrass restoration site. The perches attracted birds whose waste deposited 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 co-author Amanda Rodewald, a professor and senior director of the Center for Avian Population Studies at the Cornell Lab of Ornithology.
Rochester-area songbirds show seasonal differences
A related Functional Ecology field study examined a local part of that larger question. Cornell researchers collected waste from 151 individual songbirds representing 31 species at Braddock Bay Bird Observatory on Lake Ontario, near Rochester. The published paper places the sampling in fall 2021 and spring 2022. Researchers measured nutrients in the waste.
The team found that waste was richer in nitrogen in spring, when birds tended to eat more insects. In fall, when fruits and seeds were more prominent in their diets, the waste contained more carbon. Body size and seasonal changes in fat reserves also helped explain differences among birds.
The field study directly measured waste from birds at one stopover site during brief sampling periods. Its authors said that limits how broadly the measured rates can be applied. Phosphorus was measured in only 31 samples, and the researchers noted that capture and handling could have affected how much waste the birds released.
The broader literature review identifies pathways by which declining bird numbers could affect ecosystems. BirdLife International data show declining populations for 45% of the 1,843 migratory bird species it assessed globally.
Duvall said the combined research could help scientists make better estimates of how changes in bird populations affect nutrient cycling, plant growth and other ecosystem processes.
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