We used an allometric approach to compare the minimum nitrogen requirements (MNR) and the total endogenous nitrogen loss (TENL) of nectar- and fruit-eating birds with those of omnivorous birds. These two parameters were 4x higher in omnivores than in nectarivores and frugivores. In nectarivorous-frugivorous birds, MNR was 152.8 mg N kg(-0.76) day(-1); in omnivorous birds, it was 575.4 mg N kg(-0.76) day(-1). Similarly, TENL was 54.1 mg N kg(-0.69) day(-1) in nectarivores-frugivores, and 215.3 mg N kg(-0.69) day(-1) in omnivores. The residuals of the allometric relationships between TENL and MNR and body mass were positively correlated, which suggests that a large proportion of the interspecific variation in MNR is explained by variation in TENL. Although our results show that nectar- and fruit-eating birds have low nitrogen requirements, the mechanisms that these animals use to conserve nitrogen remain unclear.
Tsahar, E.; Ara, Z.; Izhaki, I.; and del Rio, Carlos Martinez (2006). "Do Nectar- and Fruit-Eating Birds Have Lower Nitrogen Requirements Than Omnivores? an Allometric Test." Auk 123.4, 1004-1012.