Uric acid is not very soluble in water

A single molecule of uric acid eliminates twice the amount of nitrogen as does a molecule of urea (see Figure 51.3), but uric acid is not very soluble in water. Therefore it tends to precipitate out of solution and form a colloidal suspension in the urine and is excreted as a semisolid (for example, the whitish material in bird droppings). Because of the precipitation of the uric acid out of solution in the excretory system, water can be reabsorbed from the urine and thereby reduce the water loss necessary to excrete the nitrogenious waste. Animals that excrete uric acid are uricotelic, and they include insects, reptiles (including birds), and some amphibians. Many uricotelic species live in very arid environments.