Ammonia ($\text{NH}_3$) is the most toxic nitrogenous waste and is produced only by animals with ready access to large amounts of water for dilution — including aquatic insects and fish. It requires minimal energy to produce but must be excreted immediately. Urea is less toxic and excreted by mammals and adult amphibians; uric acid is least toxic, nearly insoluble, and excreted by birds, reptiles, and terrestrial insects to conserve water.