There is no store for nitrogen-having compounds as there is for carbohydrate (glycogen)   or lipids (triacylglycerol).  Thus nitrogen ingested in excess of what is required through the organism has to be excreted.  The excess nitrogen is first transformed into ammonia and is then excreted from living organisms in one of three ways.  Various aquatic animals simply excrete the ammonia itself straightly into the surrounding water. The Birds and terrestrial reptiles excrete the ammonia in the form of uric acid, though most terrestrial vertebrates convert the ammonia into urea before excretion.  These three classes of organisms are called as: ureotelic, ammonotelic and uricotelic, respectively.