Why is most bird poo white, even though birds eat food of many different colours?

The answer has two parts. One is that birds have to get rid of excess nitrogen in their diet. The other is that bird poo does actually come in different colours.

Nitrogen is an element in all proteins. Sometimes there is too much of it in what you eat, meaning you need to get rid of unwanted nitrogen. Evolution has settled on three main ways to do this and each links to how much water there is in the local environment.

First: in a lake, river, ocean or other body of water, fish and turtles will add three atoms of hydrogen to every one atom of nitrogen.That gives you ammonia, which dissolves easily in water.

Second: creatures living on land that have access to lots of water get rid of their nitrogen by making a chemical that is fairly soluble in water – urea. It’s a chemical that has two nitrogen atoms, four hydrogen atoms and one atom each of oxygen and carbon.

And third, we come to creatures that live in very dry areas – and birds. Birds have to be as light as possible, so they don’t want to drink (and carry) lots of water. These creatures get rid of their excess nitrogen as the chemical called uric acid. Uric acid has four nitrogen atoms joined to 12 atoms of carbon, hydrogen and oxygen. It’s also white, mostly insoluble in water, and is excreted from the animal in the form of a dryish paste.

So is bird poo always white? Well, it’s actually a mix of their regular faeces, and a topping of some uric acid. If you look at the poo from your basic domestic chicken, it’s a kind of off-white fibrous lump, topped with some white uric acid.

But of course, if the bird eats a lot of mulberries or blackcurrants, then the overwhelming colour of its poo (thanks to its faeces) will be purple or reddish.

For birds then, the combination of uric acid and solid matter is a great evolutionary method for dumping their egg-cess nitrogen.