A procedure is described for the accurate determination of the maintenance nitrogen requirement (MNR) of small granivorous birds. When used with the zebra finch (Taeniopygia guttata), it yielded a MNR of 403 mg kgW'0.75 d'1. This is lower than most other passerines so far measured and more similar to some nonpasserine species. Similarly, the value for endogenous nitrogen loss estimated for the zebra finch (153 mg kgW'0.75 d'1) is less than that for passerines in general but higher than the nonpasserine value. We suggest that the low MNR of the strictly granivorous zebra finch is primarily an adaptation to seed diets in which high'quality protein is a limiting factor. Comparison with a wider range of species reported in the literature was restricted because inappropriate methods have been used to estimate MNR in many cases, including the use of growing or reproducing birds and the assumption that maintenance of body mass necessarily coincides with maintenance of zero nitrogen balance.