Sources of Assimilated Protein in a Specialized Tropical Frugivorous Bird, the Yellow-throated Euphonia (Euphonia hirundinacea)

Abstract

ABSTRACT.— 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. ...