Birds, insects, and many reptiles excrete nitrogenous waste in the form of uric acid, which see concept 44.2 (page 980)

Forms solid that are relatively insoluble and nontoxic

Uric acid is  product of the metabolic breakdown of the purine nucleotides. It is considered as a normal component of urine. In birds and reptiles the evolved the ability to convert toxic ammonia into uric acid or guanine rather than urea. These organisms are called as uricothelic organism because they have the ability to convert toxic ammnonia to uric acid instead of urea. Uric acid is less toxic than ammonia.