Most frugivorous birds must complement their diet with insects to fulfill their protein requirements; species that rely almost exclusively on fruits are rare. We used nitrogen (¹⁵N:¹⁴N) stable-isotope analysis of whole blood to quantify and compare the use of insects and fruits as sources of assimilated nitrogen in a specialized frugivorous bird, the Yellow-throated Euphonia (Euphonia hirundinacea). Nitrogen stable-isotope analysis is particularly useful for examining the relative contribution of animal and plant products as sources of assimilated nitrogen, because there is trophic enrichment of the heavier isotope in higher trophic levels of food webs. We found that most assimilated nitrogen was derived from fruits (76–100%), with a slight increase in the contribution of fruits as their abundance increased. In comparisons of bird species, δ¹⁵N values of Yellow-throated Euphonia were lower than those of insectivorous and piscivorous species but similar to the values found in seed-fruit eaters. Our results suggest that the dependence on fruits as a source of assimilated nitrogen in Yellow-throated Euphonia ranged from predominant to almost exclusive during the period of our study (February–August). Total dependence on fruits as a source of assimilated nitrogen is not a common phenomenon among birds, but it may be favored in the tropics, where fruits are more abundant and diverse.