Birds eat all sorts of things, from juicy red berries, and tender green leaves, to crunchy brown insects and everything in between. So why is their poo always white?
To find out what’s going on we need to understand what bird poo is made of.
Every organism that consumes food needs to get rid of waste material and products that are not used by the body.
The elimination of undigested food is called defecation, while the elimination of the waste products from metabolism is called excretion. In humans, defecation happens via the anus with the passing of faeces, while excretion happens via the urinary tract and the passing of urine. Some waste products are eliminated from elsewhere via sweat, tears, or vomit, but for the purposes of this little physiology lesson we’re going to concentrate on the stuff that comes out of the rear end.
In humans and other mammals, about 95% of urine is water. The rest consists of salts, trace amounts of protein and hormones, and urea. Urea is a toxic, organic compound, easily soluble in water, that carries the waste nitrogen produced during the metabolic process. It takes little energy to synthesize urea in our cells, so as long as we drink enough water, excretion via urination is not a problem.
Birds, however, don’t produce urea. Instead, excess nitrogen is excreted as uric acid, which is a non-toxic, white paste, that doesn’t easily dissolve in water. It takes a lot more energy to produce uric acid than urea, but it is a price worth paying because, for birds, conserving water is more important than conserving energy.
An embryo inside an egg also needs a way to get rid of its metabolic waste. While a fish or amphibian embryo can pass water-soluble nitrogen compounds through the outer membrane of its egg, a reptile or bird embryo must store its waste inside its shell. If a bird produced urea, it would eventually poison itself, so it manufactures uric acid instead.
The most likely explanation of why the ancestors of birds and reptiles evolved to do this, despite the high energy cost involved, is so that they could lay hard-shelled eggs on land, and open up more environments in which new species could survive.
For adult birds, saving water also keeps their weight down, which means they expend less energy when flying. Conserving water is also important during migration, when birds can be far away from fresh water sources for many days.
Unlike humans, birds eliminate their waste products through one hole, the cloaca. The cloaca, also referred to as the vent, is also used for reproduction and laying eggs, and it has been suggested that in some species it is used for temperature regulation.
This means that birds’ droppings are a mixture of faeces and urine, although they are expelled at the same time, they are produced by different systems and do not mix before leaving the body.
The whiteness of a bird’s poo depends on its diet. For those species that have a protein rich diet very little faeces will be produced. Birds that eat more fruit, grains, and other plant material will produce more faeces and if you’ve ever examined a bird poo closely, you’ll often see a small dark or greenish centre. It may even be purple in birds that eat lots of berries.
The uric acid in bird poo is blamed for everything from the corrosion of buildings to the damage to cars’ paint. It has even been suggested that uric acid is the reason some urban pigeons have missing legs and feet. In Japan the Japanese bush warbler’s excrement, known as Uguisu no fun, is mixed with rice bran, and applied to the face. The facial, sometimes called the Geisha Facial, is said to work due to its high concentration of uric acid.
However, in the 1960s, Bob Folk, a professor at the Jackson School of Geosciences, claimed uric acid wasn’t present in birds’ droppings at all. He analysed samples from 17 species and found no evidence of the substance, publishing his findings in a paper entitled Spherical urine in birds: petrography, in Science magazine, the journal of the American Association for the Advancement of Science.
His conclusions were quickly refuted by a group of scientists who used the same X-ray diffraction technique as Folk and found evidence for uric acid in the droppings of budgerigars.
The question lay dormant for decades until 2018, when Nick Crouch, a post doctoral fellow studying bird evolution and biodiversity at The University of Texas at Austin had a conversation with Folk. With the advancements in the technology for analysing the results of X-ray diffraction, Crouch decided to take another look for himself.
He analysed samples of droppings from a diverse range of birds including a variety of diets as well as flightless birds. Although his analysis found evidence of ammonium urate, struvite, and two unknown compounds, none of the samples produced an X-ray diffraction pattern consistent with uric acid.
Based on his findings along with research from other scientists, Crouch concluded that bacteria inside a bird’s gut breaks down uric acid before it is excreted producing the unidentified substances.
So, although the received wisdom is that bird poo is white due to uric acid, the answer may not be so simple, and more research is needed until we can answer the question definitively.
