Dogs lift their legs. Cats use a litter box. And birds do their business just about everywhere. But is this whitish-colored splat only bird poop? What about bird pee?
"Their pee doesn't look like ours," said Sushma Reddy, an associate professor of ornithology at the University of Minnesota. "It's not liquid." Instead, birds drop a white paste that contains both pee and feces.
Birds have one hole to get rid of waste, called a cloaca. The cloaca is the multitool of a bird's anatomy: It leads to the urinary, digestive and reproductive tracts. It's through this hole that birds simultaneously pee and poop onto cars and unsuspecting humans.
Related: Why do parrots live so long?
Dinosaurs, the ancestors of birds, also had cloacas, as do amphibians, reptiles, some fishes and monotremes, such as platypuses. From an evolutionary perspective in birds, this one-and-done method of waste excretion likely was preserved because it was an adaptation toward flight, Reddy said. It takes a lot of energy to maintain flapping wings, so birds need to stay light and keep essential nutrients for long periods of flight and excrete the stuff they don't need quickly and efficiently as they're flying, Reddy said.
Flying helps explain the differences between bird and mammal urinary systems. Mammals get rid of nitrogen waste by converting ammonia (a toxic substance) into urea that is then diluted in liquid. This is the urine that humans and other mammals pee.
Birds convert nitrogen waste excreted by the kidneys into uric acid, which doesn't dissolve in water and comes out as a solid, as explained in a manual on "avian systems" from the University of Wisconsin-Madison's Extension Dodge County. This means birds can conserve water and don't need to carry around a heavy, full bladder.
But bird pee may be more complex than previously understood. A study published in 2020 in the Journal of Ornithology tested the chemical composition of six avian species' pee and found no traces of uric acid. Instead, the scientists found compounds made of ammonium, magnesium and phosphate. Their results indicate that an unknown process modifies uric acid right before it exits the cloaca. Future studies with more bird species could help explain this mysterious transformation.
Before reading this recent research paper, Reddy said that she and other ornithologists would have firmly stated that uric acid was the main ingredient of bird droppings dirtying car windshields. "Sometimes those things that are in textbooks are exactly what we should be testing again," Reddy said of the findings.
Reddy uses bird droppings to study DNA, so understanding the exact composition of bird pee is important for her work. Her lab uses excrement from endangered piping plovers (Charadrius melodus) as a non-invasive way to collect genetic samples that provide information about the birds’ diets, their microbiome and even diseases. DNA gleaned from pee, Reddy said, is like "a lens into all these different things that we can tell about the birds."
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Hannah Loss is a science journalist based in Boston. She covers the environment and has written for Scientific American, Sierra and Inside Climate News. Hannah graduated from Tufts University with a B.A. in English and environmental studies. She received a Master's degree in journalism from NYU's Science, Health and Environmental Reporting Program.