Ammonia excretion:

There is no store for nitrogen-containing compounds as there is for carbohydrate (glycogen)   or lipids (triacylglycerol).  Therefore nitrogen ingested in excess of what is needed 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.  Several 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, although most terrestrial vertebrates convert the ammonia into urea before excretion.  These three classes of organisms are known as: ureotelic, ammonotelic and uricotelic, respectively.