Digestion of nectar and insects by Palestine sunbirds

In nectarivorous birds, specialization for feeding on nectar has led to a simple gut structure with high sugar digestive efficiencies and rapid gut passage rates. These features of the digestive system may make digestion of more complex, protein-rich food sources, such as pollen or insects, less efficient. In this light, we hypothesized that sugar metabolizability in nectarivorous Palestine sunbirds (Nectarinia osea) would be high, whereas nitrogen metabolizability would be lower than typically found for birds. We measured glucose and fructose apparent metabolizabilities (*MCs) and transit times (TTs) in eight Palestine sunbirds offered either a 10% or a 50% mixed sugar diet. *MC for glucose (99.9% ± 0.1%) was significantly greater than for fructose (99.6% ± 0.4%; ANOVA; P < 0.001). TT for the 10% sugar diet (26.3 ± 10.1 min) was significantly shorter than for the 50% sugar diet (47.0 ± 7.8 min). We measured nitrogen true metabolizability (MC) and TT in Palestine sunbirds offered a daily fruit fly intake of either 40 or 200 flies. Nitrogen MC was not significantly different between diets, and average MC for both diets was 58.5% ± 8.5% (n = 8). TT was not significantly different when birds ate 10 flies (50.1 ± 13.6 min) than when they ate 50 flies (48.5 ± 16.5 min). The high sugar *MC and relatively rapid TT of nectar in Palestine sunbirds are similar to those found for other nectarivorous species. Transit times of insect material are longer that those found in small insectivorous species. However, MCs of insect material are lower. Thus, even though sunbirds consume easily digestible soft-bodied insects, they are less efficient at extracting protein than nonnectarivores.