theperfect.mismatch

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.

Most aquatic vertebrates excrete nitrogenous wastes in the form of ammonia, which is highly toxic but very soluble and easily gotten rid of if water is in ample supply. Uric acid excretion may have first developed in the first vertebrates to evolve shelled, fully terrestrial eggs. Such eggs must retain the waste products produced by embryonic metabolism within the shell until hatching. Toxic, soluble ammonia would soon poison a developing embryo, while non-toxic, insoluble uric acid can simply be stored inside the shell as long as necessary. In developing live birth, mammals may have switched to back to a more soluble compound, urea, so that embryonic waste products could be diffused into the blood stream of the mother and thus excreted.