Sugar and nitrogen digestive processing does not explain the specialized relationship between euphonias and low quality fruits

In the Neotropical region, euphonias (Euphonia spp., Fringillidae) are the
quintessential example of specialized bird frugivores, making the bulk of
feeding visits to certain mistletoes (Phoradendron spp., Santalaceae) and
epiphytes in the genus Rhipsalis (Cactaceae), whose fruits have high water
and low sugar and protein concentrations. Surprisingly, a mechanistic
explanation for such specialized, otherwise rare, relationships is
lacking. Using captive birds and artificial diets, we contrasted euphonias
with frugivorous tanagers in the genus Thraupis (Thraupidae), which rarely
eats Rhipsalis fruits, to test the hypothesis that the digestive capacity
of euphonias entails them to exploit such low-energy fruits. We expected
that compensatory feeding in response to decreasing energy density would
occur only in euphonias, whose higher reliance on fruits would entail a
lower nitrogen requirement than the tanagers. Euphonias and tanagers were
both able to compensate energy intake as sugar density decreased, and both
species had the same mass-corrected energy intake at any given sugar
concentration. Similarly, euphonias and tanagers did not differ in
mass-corrected maintenance nitrogen requirement. Therefore, the
physiological traits we investigated do not explain euphonia´s
specialization on Rhipsalis fruits. The fast rates of fruit passage
typical of specialized avian frugivores as euphonias that entail the
processing of a large volume of fruits, and the putative better abilities
of such birds to deal with secondary compounds likely present in Rhipsalis
fruits are other possible mechanisms that should be considered in future
studies to unveil the mechanisms underlying the intriguing specialized
relationships between euphonias and certain fruits.