Which of the following adaptations is primarily used by birds and insects for water conservation?

The correct answer is uric acid, which serves as an effective adaptation for water conservation primarily in birds and insects. This waste product is less toxic than ammonia and can be excreted as a semi-solid paste, thereby minimizing water loss. Since birds and many terrestrial insects often inhabit environments where water is scarce or where they cannot frequently access liquid water, this adaptation allows them to conserve moisture and maintain hydration.

In contrast, urea is more soluble in water and requires significant amounts of water for excretion, making it less efficient for water conservation in these organisms. Ammonia, while highly toxic, is typically excreted by aquatic animals that have the advantage of being surrounded by water, allowing them to dilute this waste product easily. Glycogen, while important as a form of energy storage, does not play a direct role in water conservation for these organisms. Therefore, uric acid is the most efficient means by which birds and insects reduce water loss while managing nitrogenous waste.