Explain why :
Mammals are ureotelic, but birds are obliged to be uricotelic.
Mammals normally get required amount of water. Urea present in water can be stored in the urinary bladder that does not disturb the metabolism of the body. Birds are uricotelic as water is not easily available in excess. Urinary bladder is absent in birds and ureters open into cloaca also absorbs water. The body of bird must be kept light for the flight mechanism thus the presence of water will be disadvantageous for them. Thus they are uricotelic as uric acid is concentrated while urea needs more amount of water to dissolve,