thick, insoluble solution of metabolic wastes formed in the kidneys for expulsion through the vent at the end of the cloaca; a more concentrated substance in birds (and reptiles and insects) than the urea formed in mammals and requires considerably less water to form or expel, thus allowing much greater retention of water for use in the body and eliminating the need for a bladder to hold water for excretion; in a developing embryo uric acid is stored in the allantois (one of the shell membranes) and is discarded after hatching; also urates