Birds excrete that white pasty stuff instead of separately urinating and defecating like mammals do. So they don't exactly urinate, but they do something like it. All animals have to get rid of the nitrogen that is left over after breaking down proteins. Mammals turn it into urea and urinate it out. Birds and reptiles turn it into uric acid. Birds have a chamber called a cloaca where digestive wastes, uric acid, and eggs or sperm go before leaving the body. While the uric acid is in the cloaca, birds can absorb some of the water back. This helps them to conserve water. Some sea birds can even drink sea water and excrete the extra salt out of glands near their eyes.

Why do you think birds need to avoid having a full bladder? Think about the costs of flight.

Birds, like all animals, have to get rid of their nitrogenous wastes somehow. When animals digest proteins and other nitrogen-containing compounds, one of the waste products is ammonia, which is toxic. Ammonia is so toxic, in fact, that it has to be kept pretty dilute in order to protect the animal. So the simplest solution is to excrete lots and lots of very dilute urine with ammonia in it. This is what lots of aquatic animals do, since they don't have to conserve water.

Mammals, for the most part, don't have access to an unlimited supply of water, so don't want to have to urinate gallons and gallons of ammonia every day.Instead, we convert the ammonia to the slightly less toxic compound urea. We can concentrate urea more than ammonia, so we can urinate less and conserve water.

Reptiles and birds, on the other hand, convert ammonia to uric acid, which is even less toxic than urea. In fact, uric acid doesn't need any water at all -- they can excrete it as a solid and conserve as much water as possible. If you've ever had the good fortune to take a good look at bird droppings, you've probably noticed that they tend to have black chunks (the feces) in a white pasty substance. This white stuff is mostly the uric acid. If you want some bonus vocabulary to impress your friends, here goes:

ammonia-excreting animals are 'ammonotelic', mammals and anything else that uses urea are 'ureotelic', and the birds and reptiles are 'uricotelic.'

Birds crystallize their urine and excrete it, along with feces, out their cloacas (a hole that has been turned inside-out in mammals, including the urinary excretory orifice, the anus, and, in females, the vaginal opening).

Yes, birds excrete urine. Birds have only one pathway to excrete any substance called a cloaca, so all the fluids and materials they excrete pass through the cloaca, usually in a mixed form.