Laura Erickson's For the Birds

One subject wisely left out of most polite discourse is bird droppings. I mainly hear about them in summer when people park their cars near a gull colony. But after several months of snow and ice, I’m nostalgic for that more summery white stuff.

The whitewash on a bird’s dropping, which is actually the bird’s urine, is the very thing that makes bird droppings so noticeable and interesting. Birds don’t have a bladder like mammals do–that would be excess baggage. Urine formed in a bird’s kidneys leaves the body fairly quickly, without accumulating. But that doesn’t explain why it’s white.

It all starts with the protein food that an animal eats. It’s broken down into smaller proteins and amino acids in the intestines and then sent around the body to provide energy and materials for growth. As the body’s main chemical factory, the liver especially uses them, producing waste chemicals laced with nitrogen. In mammals, these nitrogenous wastes form a poisonous chemical called urea, which is kicked out of the cells and sent through the bloodstream as quickly as it forms. As soon as a urea molecule reaches the kidneys, they filter it out of the blood and shoot it down to the bladder, diluted with plenty of water to keep the animal from getting poisoned. A tiny mammal embryo absorbs amino acids through the placenta from its mother’s blood. As it uses amino acids to grow, it produces urea which is filtered through the placenta back into the mother’s blood. Her kidneys clean it out, providing her with the opportunity to literally go to the bathroom for two. In the overall scheme of the universe, this system works pretty well unless you’re nine months pregnant at the Duluth Entertainment and Convention Center, where there aren’t enough stalls in the women’s restrooms.

But a bird embryo grows inside an egg without a placenta handy to filter out wastes. As a tiny bird grows, absorbing nutrients from the yolk, it produces plenty of nitrogenous wastes, but it’s stuck with them until it hatches. If those wastes were in the form of urea, it’d be poisoned inside the egg. So, instead, it forms a different nitrogen chemical altogether, called uric acid. Uric acid conveniently precipitates into a white crystal solid. As the tiny embryo’s kidneys filter out this uric acid, it accumulates in a tiny chamber of the egg called the allantois–the egg’s bathroom. The baby bird stays clean and safe even without disposable diapers.

Once a bird hatches, that non-toxic uric acid allows it to produce droppings that are concentrated and relatively dry–well, except for those pesky gulls. This concentrated crystalline urine allows birds to survive for fairly extended periods without any drinking water. Thats why there are more birds than mammals in habitats where running water is at a premium, from deserts to the frozen Northland. When it comes right down to it, if it weren’t for those strange white droppings, there wouldn’t be any birds to talk about on a frozen morning in the Northland.