Montana Public Radio

Everywhere You Look, Birds Are Dropping Clues

Mar 9, 2021

With the joy of birding comes the fear of getting dropped on. Recently, my seven-year-old daughter carefully watched a pair of Canada geese sitting on an old ponderosa pine snag. She was looking for an owl’s nest below, turned to me and wondered: “Why is bird poop white?”

Wanting to make this a memorable homeschool experience during our pandemic times, I found fascinating answers and unearthed a recent, surprising revolution in the world of bird poop research.

First, unlike mammals, birds don’t pee. This is no news to bird lovers, but fascinating to a political science PhD who apparently missed that fact in elementary school biology. Instead, birds poop solids and liquids in one dropping. These appear different between species. A thick, three-inch tubular mass of grey-green color is probably from a pheasant; a short, dark worm-like structure with a white coating containing ant exoskeletons could be a dropping from a woodpecker. Some birds like owls, eagles, gulls, or flycatchers spit out part of what they eat as pellets. Dissecting those is another super fun homeschooling activity in pandemic times. Depending on what the bird eats, the bird poop is made out of a more solid brown part and a more liquid white part. For example, birds that mainly eat fish like our great blue heron, gulls, or osprey poop almost only white. This “white gold,” or guano, from seabirds off the Peruvian coast was a highly demanded commodity as agricultural fertilizer in the United States in the 19th century.

But why is the bird poop white? Commonly, bird researchers assumed the white part to be uric acid – the urine component produced by birds during their water and food digestion. In mammals, like us, kidneys play a key role in balancing water intake and output. The birds’ kidneys and their lower intestine do the job without needing to store the liquid in a heavy bladder. Some birds pass this single white-brown dropping  rather fast. The blackcap, an Old World warbler, completes digestion of a meal of berries in just 12 minutes! On the opposite end, the haotzin, a tropical bird, has to rest all day to ferment and digest the leaves it likes to eat – but it is also a poor flyer.

In early 2020, the Journal of Ornithology published a study that turns the common wisdom that the white in bird poop is uric acid upside down. A research team from the University of Texas at Austin found that the white in the poop is something much more mysterious. They tested fresh poop from eight different bird species: chicken from their neighbor’s yard and seven species from the Austin Zoo: emu, ostrich, helmeted guineafowl, great horned owl, blue-and-gold macaw, and two small parrot species: a jenday conure and a rainbow lorikeet. Instead of uric acid, they found ammonium urate, struvite (better known as magnesium ammonium phosphate), and two unknown compounds! Bacteria in the bird’s gut, the paper concludes, breaks down the uric acid into these chemicals, giving them the white color before exiting the bird’s cloaca.

The consequences of these findings could be potentially groundbreaking for bird poop research. Do birds balance salt and water differently than previously understood? Why do embryos in eggs remain undamaged by the ammonium urate? And why, if not for the uric acid, is it so hard to remove bird poop from windows? These are great questions I can leave to bird researchers. Fortunately, my daughter seems satisfied with the answer I found to her question, for now.

'Field Notes' is produced by the Montana Natural History Center.

(Broadcast: "Fieldnotes," 3/9/21 and 3/12/21. Listen weekly on the radio Tuesdays and Fridays at 4:54 p.m., or via podcast.)