Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores

Ella Tsahar, Carlos Martínez Del Rio, Ido Izhaki, Zeev Arad

Research output: Contribution to journalArticlepeer-review


We measured minimal nitrogen requirements (MNR), total endogenous nitrogen loss (TENL) and the effect of protein and water intake on the nitrogenous waste composition in two frugivorous bird species: yellow-vented bulbuls Pycnonotus xanthopygos and Tristram's grackles Onychognathus tristrami. The nitrogen requirements of both species were much lower than expected for their body mass. The two species differed in the composition of the nitrogenous waste that they produced. The grackles were uricotelic, and the chemical composition of their nitrogenous waste products was relatively independent of water and protein intake. In contrast, the bulbuls were 'apparently ammonotelic'. Their ammonotely was related to low protein intake and high water flux, and was the result of post-renal urine modification. We suggest two non-exclusive mechanisms for the post-renal modification of urine in these birds: bacterial catabolism of uric acid and reabsorption of uric acid in the hindgut. As uric acid functions both as a nitrogenous waste product and as an antioxidant, birds might benefit from its reabsorption.

Original languageEnglish
Pages (from-to)1025-1034
Number of pages10
JournalJournal of Experimental Biology
Issue number6
StatePublished - Mar 2005


  • Ammonia
  • Frugivores
  • Nitrogen balance
  • Nitrogen requirements
  • Onychognathus tristram
  • Pycnonotus xanthopygos
  • Tristram's grackle
  • Uric acid
  • Yellow-vented bulbul

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Physiology
  • Aquatic Science
  • Animal Science and Zoology
  • Molecular Biology
  • Insect Science


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