Why are there so few exclusively frugivorous birds? Experiments on fruit digestibility

Birds of 6 non-exclusively frugivorous passerine species were kept on 6 exclusive single-species fruit diets. Birds and fruits originated from the same Mediterranean scrub community in Israel. Most birds lost weight steadily, in spite of an increased rate of pulp intake. Weight loss was greater in small birds and in birds which are more frugivorous in nature than others. Weight loss increased as protein digestibility decreased, and decreased with the proportion of protein in diet, hence buckthorn Rhamnus palaestinus proved the worse diet, and gardrobe Osyris alba the best. Most birds ingested more energy and more protein than required for maintenance, but metabolized and digested less than required. Fleshy fruits are nutritionally adequate but they probably contain agents that reduce the efficiency of the birds' N metabolism, leading to the inability to subsist on an exclusive fruit diet. Plants improve seed dispersal, by forcing birds to break frugivory by bouts of insectivory, and move away rather than stay on a rich clump of fruits and deposit all seeds underneath parents. -Authors