Do avian frugivores absorb fruit sugars inefficiently? How dietary nutrient concentration can affect coefficients of digestive efficiency

Fruit-eating birds are thought to inefficiently digest and/or assimilate nu trients from fruits. Contradicting this established premise, I found that C edar Waxwings Bombycilla cedrorum and three thrush species absorbed fruit s ugars efficiently, comparable to levels of sugar assimilation shown by nect ar-feeding birds. Relatively low sugar digestive efficiencies by birds eati ng Lonicera morowii fruit could be explained; by the especially low sugar s olute concentration of this fruit's pulp. Sugar digestive efficiency is a p roportional measure of sugar extraction from ingested sugar solutions. When most ingested water is excreted with unabsorbed sugars, digestive efficien cy will decline as fruit sugar concentration decreases if sugars are reduce d to relatively constant, minimum concentrations in excreted water. Under t his circumstance, fecal nutrient concentration may be more appropriate for comparisons of how well animals utilize nutrients from foods that differ in their concentrations of dissolved or dispersed nutrients. Efficient absorp tion of fruit sugars by Cedar Waxwings and thrushes contradicts the premise that sugary, bird-dispersed fruits challenge the digestive systems of avia n frugivores so that nutrient utilization is impaired. Because simple sugar s (glucose and fructose) are readily assimilated, digestive processing of s ugary fruits is more rapid than processing of foods containing complex nutr ients. Thus, fruits represent accessible, energy-rich foods from which nutr ients are efficiently absorbed by avian frugivores.