Uric Acid: The Main Nitrogenous Excretory

any, uric acid. The biological literature contains abundant evidence that most excretory nitrogen of birds is in the form of uric acid or its salts (2). Most of Folk's data on the white part (the urine) of bird droppings are not inconsistent with this generalization. (i) Folk finds that bird droppings exhibit only part of the x-ray analysis pattern typical of uric acid or two of its salts. We would like to know why Folk emphasizes the heterogeneous composition of the droppings that he analyzed but rejects the idea that the x-ray peaks seen are for as yet uncharacterized salts of uric acid or obscure some of the peaks typical of uric acid, or both. Folk himself says that ultraviolet spectrophotometric analyses demonstrate the presence of the urate radical in his samples. (ii) Folk argues that bird droppings dissolve in water or weak acids and therefore cannot be uric acid. Even so, the original dried droppings could have been some urate salt that dissolved and then recrystallized as uric acid. Furthermore, it seems odd for Folk to feel that the addition of water alters the composition of bird urine, when one considers that he used "air-dried" excrement that itself is quite modified from the condition in which it entered the cloaca from the ureters and passed retrograde into