Most birds are uricotelic. An exception to this rule may be nectar-feeding birds, which excrete significant amounts of @entity3265 under certain conditions. Although @entity3265 is toxic, because it is highly water soluble its excretion may be facilitated in animals that ingest and excrete large amounts of water. Bird-pollinated plants secrete @entity1735 - and water-rich floral nectars that contain exceedingly little protein. Thus, nectar-feeding birds are faced with the dual challenge of meeting @entity1568 requirements while disposing of large amounts of water. The peculiar diet of nectar-feeding birds suggests two hypotheses: (1) these birds must have low protein requirements, and (2) when they ingest large quantities of water their primary @entity1568 excretion product may be @entity3265 . To test these hypotheses, we measured maintenance @entity1568 requirements (MNR) and total endogenous @entity1568 losses (TENL) in three hummingbird species (Archilochus alexandri, Eugenes fulgens, and Lampornis clemenciae) fed on diets with varying sugar, protein, and water content. We also quantified the form in which the by-products of @entity1568 metabolism were excreted. The MNR and TENL of the hummingbirds examined were exceptionally low. However, no birds excreted more than 50% of @entity1568 as @entity3265 or more @entity1568 as @entity3265 than urates. Furthermore, @entity3265 excretion was not influenced by either water or protein intake. The smallest species (A. alexandri) excreted a significantly greater proportion (>25%) of their nitrogenous wastes as @entity3265 than the larger hummingbirds ( approximately 4%). Our results support the hypothesis that nectar-feeding birds have low protein requirements but cast doubt on the notion that they are facultatively ammonotelic. Our data also hint at a possible size-dependent dichotomy in hummingbirds, with higher @entity3265 excretion in smaller species. Differences in proportionate water loads and/or postrenal modification of urine may explain this dichotomy.

Are hummingbirds facultatively ammonotelic? XXXX excretion and requirements as a function of body size.

@entity1568 :: ('MESH:D009584', 'Chemical') :: ['nitrogen'] @entity1735 :: ('CHEBI:16646', 'Chemical') :: ['carbohydrate'] @entity3265 :: ('MESH:D000641', 'Chemical') :: ['ammonia']
@entity1568 :: (MESH:D009584,Chemical) :: ['Nitrogen']
