Birds, unlike mammals, do not excrete liquid urine, which often leads to confusion about the nature of their waste. All waste products are combined and expelled through a single opening called the cloaca. The substance people commonly observe is a mixture of solid and semi-solid materials.
The Three Components of Bird Droppings
What appears to be a single dropping is composed of three distinct parts that merge upon excretion. The dark, often coiled portion is the feces, which is solid, digested food waste from the intestinal tract. Its color typically ranges from brown to green, depending on the bird’s diet.
The white material is called urates, the bird’s equivalent of concentrated urine. This pasty substance is made up of uric acid crystals and is responsible for the characteristic white color. A small amount of clear liquid urine is also present, which is the watery component that mixes with the feces and urates.
The Biological Reason for Uric Acid
The white material is uric acid, the primary way birds eliminate nitrogenous waste from the body, classifying them as uricotelic organisms. Mammals convert their nitrogenous waste into urea, which requires a large volume of water to flush out as liquid urine. Birds use uric acid as an adaptation driven by two primary evolutionary pressures: water conservation and flight.
Uric acid is poorly soluble in water, allowing birds to package nitrogenous waste into a semi-solid paste with minimal water loss. This is beneficial for flying creatures, as it prevents the need to carry excess water weight that a liquid bladder would require. The kidneys filter the blood, and the resulting uric acid passes into the cloaca, where the body reabsorbs as much water as possible before excretion.
This metabolic pathway, though more energy-intensive than producing urea, drastically reduces the risk of dehydration. Furthermore, the low toxicity of uric acid allows it to be safely stored within the bird’s egg during embryonic development. It precipitates into a solid form inside the egg, preventing the buildup of soluble, toxic waste that would harm the developing embryo.
When Bird Waste Colors Change
The color of the droppings can change based on diet and overall health, serving as a simple indicator of a bird’s well-being. Diets rich in certain fruits or vegetables can temporarily tint the fecal component; for example, red berries cause a reddish hue. Consuming high-water-content foods like melons results in a larger volume of the clear liquid urine component.
If the urates themselves change color, it can signal a health problem. A persistent change from the normal white to yellow or bright green may suggest liver disease, such as hepatitis. The presence of red or black in the droppings, if not attributable to food, may indicate internal bleeding or heavy metal poisoning.