My analysis based on Daniel Atkinsons work, professor in biochemistry (see references below), is that ureotelism is saving 3/4ths water compared to ammonotelism. Is this correct? What do people here think? Here is my analysis: https://telegra.ph/Urea-and-uric-acid-as-adaptation-to-conserve-water-08-27.
Land living animals therefore neutralize the bicarbonate by deprotonating the NH4+, by turning it into a much weaker base, urea (or uric acid in the phylogenetic clade birds are in. ) This forces the proton onto the bicarbonate instead, and the bicarbonate is excreted as CO2 gas, using no water. The nitrogen is also excreted at a lower cost (osmotically) than with ammonia. Urea has two nitrogens per molecule, so, half the cost osmotically, and the uric acid vector forms a non-soluble paste that has no osmotic water cost. The water saved by ureotelic animals (those excreting nitrogen as urea) is therefore 3/4th, ureotelism removes the osmotic cost of excretion for all bicarbonate (100%), and half the ammonia (50%), 100%*1/2 + 50%*1/2 = 75% of those end products from protein catabolism, i.e., only 1/4th water needed.
Atkinson, D. E., & Bourke, E. (1987). Metabolic aspects of the regulation of systemic pH. American Journal of Physiology-Renal Physiology, 252(6), F947–F956. https://doi.org/10.1152/ajprenal.1987.252.6.f947