Comparison of the cost of short flights in a nectarivorous and a non-nectarivorous bird

Although most birds are accustomed to making short flights, particularly during foraging, the flight patterns during these short periods of activity differ between species. Nectarivorous; birds, in particular, often spend time hovering, while non-nectarivorous birds do not. The cost of short flights is likely therefore to differ between nectarivorous and non-nectarivorous birds because of the different energetic contributions of different flight types to the behaviour. The C-13-labelled bicarbonate technique was used to measure the energy cost of short flights in the nectarivorous Palestine sunbird Nectarinia osea (mean mass 6.17+/-0.16 g, N=8) and the non-nectarivorous; starling Sturnus vulgaris (mean mass 70.11+/-1.11 g, N=9). The technique was initially calibrated in five individuals for each species at temperatures ranging from 1 to 35degreesC, by comparing the isotope elimination rate to the metabolic rate measured simultaneously by indirect calorimetry. The cost for short intermittent flight was then measured by encouraging birds to fly between two perches at either end of a narrow corridor (perch distance for sunbirds, 6 m; for starlings, 5 m), and measuring the amount of isotope eliminated during the flight. The isotope elimination rate was interpolated onto the calibration equation to predict flight cost, as a direct calibration could not be performed during flight. Mean energy expenditure during flight was 1.64+/-0.32 W in sunbirds, while in starlings the flight costs averaged 20.6+/-0.78 W. Energy cost of flight relative to basal metabolic rate was substantially greater in the starling than the sunbird. Phylogenetic analysis of different modes of flight in these and additional species suggests that differences in flight behaviour may cause these elevated costs in slow flying non-nectarivores such as starlings, compared to birds that are more prone to short intermittent flights like the sunbirds.