Seeing bird droppings on my car hood is irritating. Since there are no trees nearby, it's clear that it must have dropped while flying, but it's perplexing that it happened to fall on my car from the vast sky.
While bird droppings that fall from the sky can be a nuisance on land, they present an interesting research topic that can consider ecological impacts in the sea. Japanese scientists captured attention in academia by discovering birds that only relieve themselves while flying.
Professor Katsufumi Sato and Dr. Leo Uesaka from the Atmosphere and Ocean Research Institute at the University of Tokyo stated on the 19th in the international journal Current Biology that they confirmed "the uninhabited island's shearwater (scientific name Calonectris leucomelas) only defecates while flying and repeats this process every 4 to 10 minutes."
◇Researching the takeoff process with mounted cameras
The researchers noted that thanks to this unique habit, the birds can keep their bodies clean and help the sea below. The droppings of seabirds contain abundant nitrogen and phosphorus, essential for life, which can nourish plankton, the primary producers of the marine ecosystem. In fact, the droppings accumulated on the island are used as fertilizer for growing crops.
The researchers did not study the defecation habits of shearwaters from the beginning. Dr. Uesaka said, "I was researching how seabirds take off from sea level." The shearwater has long and narrow wings, which are advantageous for gliding in the sky. However, they are not suitable for flapping. The University of Tokyo researchers attached small cameras the size of erasers to 15 shearwaters to learn how they flap their wings and take off while floating in the sea.
The cameras attached to the shearwaters captured 200 defecation events. The researchers confirmed that the birds almost exclusively defecate while in flight. They found that the birds defecated immediately after takeoff. The researchers estimated that the birds defecate about 30g per hour, which corresponds to 5% of their body weight.
Occasionally, the birds defecated right after takeoff and returned to the water within a minute. Dr. Uesaka noted, "This suggests that the birds intentionally avoid defecating while afloat." The researchers speculate that the shearwater's unique defecation habit might help keep its feathers clean or avoid predators. They also speculated that it might be easier to defecate while flying in the sky than while floating in seawater.
◇Fertilizer to enrich the marine ecosystem
The researchers linked the shearwater's unique defecation habit to the food chain of the marine ecosystem. Scientists have extensively studied the impact of nutrients like phosphorus and nitrogen contained in bird droppings on soil. In fact, the hardened guano accumulated on coral islands or uninhabited rocky areas is widely used as fertilizer.
However, little is known about what impact it has on the vast sea far from the coast. The researchers revealed that the droppings of around 424 million shearwaters can enrich the sea below.
Dr. Ruth Dunn, a marine biologist at Lancaster University in the UK, said on that day to the scientific media Scientific American, "We know that seabirds have a significant impact on ecosystems through the food they consume, but now more and more researchers are starting to think about the effects of seabird droppings."
However, Professor Hugh Ellis from the Department of Biology at the University of San Diego noted that "the Japanese researchers discovered something no one had thought of," adding that it might be a unique habit limited to shearwaters or related species in contrast to his experience directly observing gulls.
References
Current Biology(2025), DOI: https://doi.org/10.1016/j.cub.2025.06.058