Birds excrete their nitrogenous wastes, derived mostly from the breakdown of proteins, in the form of uric acid rather than urea as mammals do. Unlike urea, uric acid is almost insoluble in water, and is excreted in the form of crystals that form a semisolid white paste. Not needing to store liquid wastes, birds lack a bladder. Instead urine passes from the ureters into the cloaca, a common chamber for the passage of digestive and urinary wastes, as well as for reproductive products. A bird dropping usually contains both white uric acid crystals, and a concentrated mass of digestive wastes such as insect cuticle or seeds.