A comparative analysis of nutritional quality of mixed and exclusive fruit diets for Yellow-vented bulbuls

Yellow-vented Bulbuls (Pycnonotus xanthopygos) were fed a mixed diet of four fruit species (Rhamnus alaternus, Lonicera etrusca, Rubia tenuifolia, Ephedra aphylla) and diets containing each of these four fruits separately. A mixed-fruit diet proved no more nutritious than a single-fruit diet, and all the birds lost weight at the same rate regardless of diet. Birds fed a mixed-fruit diet assimilated less protein than those fed exclusively on Rubia and Rhamnus fruits, and assimilated more energy than those fed exclusively on Ephedra fruits. The apparent metabolizable energy (AME) in bulbuls fed a mixed diet (0.73) was higher than in those fed on Rubia and Ephedra (0.69 and 0.61, respectively) but lower than in those fed on Rhamnus (0.82). Protein content alone does not explain loss of body mass. However, lack of specific amino acids, or high potassium to sodium ratios, may cause mass loss. In addition, a mixed diet of four fruit species may not prevent the accumulation of secondary compounds to a damaging level resulting in low protein assimilation.