Effects of sugar concentration on hummingbird feeding and energy use

We investigated the effect of sucrose concentration on the patterns of feeding, gut function, and energy management in the nectar-eating Chilean hummingbird Sephanoides sephanoides. We interpreted these results using a simple model of digestive function. The predictions of this model are: (a) Hummingbirds should exhibit 100% assimilation efficiency of sugars at all sugar concentrations; (b) Daily rates of energy intake should be positively correlated with sugar concentration; and (c) Increased sugar concentration should lead to linearly increasing meal retention times, and, therefore, to linearly increasing time intervals between meals. In agreement with the model, hummingbirds exhibited almost complete assimilation of sugars and increased meal retention times and intermeal intervals with increased sugar concentration. Hummingbirds did not, however, show any significant differences in daily energy intake when fed different sugar concentrations. Birds differed in their temporal pattern of feeding when fed solutions with sucrose solutions of contrasting concentrations. At low food sucrose concentrations (0.25 M), birds showed a burst of feeding before dark. In contrast, birds feeding on higher sucrose concentrations (0.5 M and 0.75 M) showed steadily declining feeding activity throughout the day. In addition to measuring the behavior and gut function of hummingbirds, we also measured their daily patterns of energy use using respirometry. Hummingbirds showed considerable flexibility in their patterns of energy use. The amount of energy used at night was positively correlated with the surplus of energy (intake minus diurnal expenditures) at dusk. Although birds exhibited only small variation in total daily energy budgets as a function of sugar concentration, birds feeding at the lowest sucrose concentration (0.25 M) seemed to rely on nocturnal torpor with more frequency than those fed on higher concentrations. We conclude that energy maximization is probably an inappropriate assumption for birds that are not growing, storing fat, or reproducing. We present a modification of the original model that allows assuming that birds do not maximize energy intake, but rather maintain constant rates of energy intake. We describe experiments and criteria that allow discriminating among the two models.