These seabirds poop on the fly (literally)

It wasn’t quite the eureka moment a team of scientists in Japan had set out for. Leo Uesaka, a marine biologist at the University of Tokyo, planned to study how seabirds use their legs to take flight from the ocean surface. He secured matchbox-sized cameras to the undersides of 15 streaked shearwaters (Calonectris leucomelas), a Pacific Ocean petrel species, to observe their movements. The tiny, tail-facing cameras successfully recorded information on the birds’ legs.

It just so happens that they also documented something else. “Unexpectedly, the footage captured many defecation events,” Uesaka tells Popular Science.

In the course of their scientific investigation, Uesaka and his co-author collected nearly 200 instances of the shearwaters pooping. Initially, the researchers thought of it as a funny accidental side-effect. But then they noticed a curious pattern in the smelly snippets, with implications for avian and ecosystem health. The findings are reported in a study published August 18 in the journal Current Biology.

In all 195 documented “excretion events,” only one shows a shearwater doing its business while sitting atop the water surface. In the remainder, all defecation occurred while flying, and usually within 30 seconds of lift-off. In 10 instances, birds took off, pooped, and then landed back on the water within the span of only one minute, suggesting the whole purpose of the flight was to go to the bathroom. Taken together, the observations imply there is a strong reason shearwaters avoid pooping while floating.

Shearwaters have narrow, long wings that make them great gliders over distances, Uesaka notes. However, the act of getting airborne takes quite a lot of effort, requiring them to flap their wings and run across the water’s surface simultaneously. It’s “comparable to a human’s sprinting,” he says. And shearwaters aren’t just hitting the head once or twice a day, but rather multiple times an hour.

The average bird in the study pooped more than five times every 60 minutes, at regular intervals. Imagine breaking into a sprint every time you had to relieve yourself, and then doing it every four to 10 minutes– all day, every day. The behavior implies that the benefits of not pooping while floating outweigh these hefty costs.

Clearly, something is motivating the shearwaters’ particular bathroom habits. The scientists aren’t exactly sure why the birds are so particular about pooping, but they have some ideas. First, it’s likely that defecating while taking off on a flight actually makes flying easier.

In addition to recording video, Uesaka and his co-author also collected droppings from captive birds to determine just how much poop they produce. Based on this data, the scientists estimate each bird releases about five percent of its bodyweight in excrement each hour– a substantial amount of weight. Dropping that biological cargo presumably lowers the energetic cost of the flapping needed to take flight.

This amount of poop also has ecological implications. Seabird droppings are a natural fertilizer. Many coastal and shoreline zones depend on a seasonal influx of the stuff. Uesaka’s new estimate of excretion mass per hour is much larger than previous assessments of seabird poop volume, based on land observations. This revised estimate of seabird excrement means that the animals are moving even more critical nutrients than previously thought. Having a better sense of when, where, how, and how much they defecate could improve scientific understanding of ocean food webs and nutrient flows.

But still, that doesn’t quite explain why the birds almost never seem to defecate while floating. Instead, Uesaka hypothesizes it could be a hygiene practice. Perhaps not pooping where you swim and forage helps to minimize infection risk. It may also aid the birds in avoiding predators, as a cloud of fresh waste might tip off hungry seals or sharks on the prowl. Defecating could also expose sensitive, internal cloacal tissue to saltwater– which might simply be unpleasant for the birds.

Finally, flying probably makes pooping easier. Perhaps the act of flexing and bearing down to become airborne naturally eases the process of going #2 (and #1– birds poop and pee at the same time, through the same hole). Though, at nesting sites on land, Uesaka says the birds frequently do just defecate on the ground.

To fully understand the exact forces at play behind the birds’ toilet time, more research is needed. Uesaka hopes to conduct follow-up spatial analyses and to repeat the experiment with other seabird species like albatross to determine how widespread the shearwaters’ poop patterns are.

In the meantime, the accidental defecation discovery goes to show that, “even something as unappealing as bird droppings can reveal surprising and interesting insights when studied closely,” Uesaka says.

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