Like the public comfort stations of Rome, mammals have multiple holes for excretion. Birds, amphibians and reptiles, however, are more like the old backyard outhouse -- one hole only. The composite ejectum from this hole is multicolored, and comprises three distinct byproducts of avian digestion.
All the waste -- solid, paste and liquid -- that leaves a bird's body does so through an organ called the cloaca. This multipurpose opening must also serve for other purposes, such as reproduction: semen goes from the male into the female through it and eggs come out of the female through it.
Bird poop isn't all white, but the white is the most eye-catching part of it. A close look at a bird dropping reveals that the white part is a white- or cream-colored pasty substance, which is concentrated uric acid. This is a metabolic waste product, created by the molecular breakdown of the bird's food and the absorption of usable molecules into the bloodstream. Birds convert the toxic residue, ammonia, into uric acid, which is filtered out by the kidneys. Uric acid has low solubility in water, which allows birds to get rid of metabolic waste while conserving water; this is a useful ability in situations like migrations over oceans or deserts, where drinking water is not available. The white color of this part of bird droppings is critical, as changes in it can indicate serious health problems.
Birds also excrete a clear liquid urine to rid themselves of excess water. How much of this they pass depends mostly on what they have been eating. Seed-eaters' poop will normally be firm, but may become watery if they suddenly eat a lot of fruit. This is normal and transitory, and should not be interpreted as diarrhea. Birds normally on a diet of fruit and vegetables tend to have watery poop. Stress can also cause temporary high urine excretion.
The dark part of bird droppings is the indigestible remains of whatever the bird has eaten, be it seed hulls, fruit pits or fish bones. Almost any shade of green or brown can be normal, but black accompanied by a strong odor can indicate problems. Some birds digest all, or almost all, of their food fully and may have almost no feces in their excrement, which will be almost entirely white; examples of this can be seen in bearded vultures, which dine almost exclusively on bones, and many fish-eating birds such as herons and cormorants.
What's Wrong with A Little Bird Poop?
Bird poop has been blamed for many problems, from damaged automobile paint jobs to the defacement and destruction of statues and even buildings, and most of this is attributed to the white or uric acid element of the poop. In the case of buildings and monuments, this is correct, since acid can corrode bronze, steel, stone, wood and iron. However, according to a 2011 study by Autoglym, a leading car care expert in the UK, it's not the white part of bird poop that does damage to a car, but the entire dropping -- as the sun bakes and hardens the blob, it also softens the clear coat over the paint and allows it to take on the reverse profile of the bird poop, creating a permanent dent on cooling. On the positive side, bird poop does have some beneficial aspects. Geisha girls in ancient Japan sought out nightingale poop to improve their complexions, and urea and uric acid (synthetic, rather than animal-based) is used today in many cosmetics. The high-nitrogen, nearly all-white poop of some sea birds is still collected from their rookeries and used as organic fertilizer, so to improve your leafy lettuce and other garden greens, offer to clean out the neighbor's chicken coop.