Uric acid is the chief nitrogenous waste of birds. Nitrogenous waste is a byproduct of protein metabolism. Different animals have different mechanisms for excreting these wastes, depending on their environments and evolutionary adaptations. Birds, unlike mammals, do not produce urine. Instead, they excrete uric acid as a semisolid paste, which helps conserve water since birds have a high metabolic rate and need to minimize water loss. Uric acid is less toxic and less soluble in water compared to other nitrogenous wastes such as ammonia and urea. This allows birds to excrete waste in a concentrated form without using excessive amounts of water. Therefore, option 4, birds, is the correct answer.