Quirks & Quarks

Question: Why is bird poop white?

Birds equivalent of urine is the pasty white glob that ends up on our cars when we park them under trees.
The thick, white portion of bird poop is their urine equivalent. (Sergey Ashmarin cc-by-sa-3.0)

This week's question comes from Fred Clipsham in Regina. He asks:

I just bought a new white car, which is great because when it is parked under the trees out front, it doesn't show the bird poop. Why, when the great majority of creatures have brown poop, do birds have white poop?

Brendan Boyd, a PhD student in the Biology Department at York University in Toronto explains that the white you see in bird poop is not, in fact, poop, but their equivalent of our urine.

Birds and mammals produce nitrogenous waste products that must be excreted from the body. A byproduct of this waste in both, is ammonia. Mammals, including humans, convert the ammonia to urea, which is excreted in urine. Birds convert the ammonia to uric acid, which appears as the thick, white paste we commonly think of as bird poop.

Birds do this to avoid taking flight with the weight of a full bladder. However, the centre of that white paste is often brown, which is the actual bird poop. Because birds only have one exit for their reproductive, digestive and urinary tracts — the cloaca — their pee and poop comes from the same place at the same time. So the green or brown trace you often see in the white uric acid paste is actually the equivalent of our feces.