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 Chi lean hummingbird Sephanoides sephanoides. We interpreted these results using a simple model of digestive function. The predictions of this m odel are: (a) Hummingbirds should exhibit 100% assimilation efficiency of sugars at all sugar concentrations; (b) Daily rates of energy inta ke should be positively correlated with sugar concentration; and (c) I ncreased sugar concentration should lead to linearly increasing meal r etention times, and, therefore, to linearly increasing time intervals between meals. In agreement with the model, hummingbirds exhibited alm ost complete assimilation of sugars and increased meal retention times and intermeal intervals with increased sugar concentration. Hummingbi rds did not, however, show any significant differences in daily energy intake when fed different sugar concentrations. Birds differed in the ir temporal pattern of feeding when fed solutions with sucrose solutio ns of contrasting concentrations. At low food sucrose concentrations ( 0.25 M), birds showed a burst of feeding before dark. In contrast, bir ds 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 meas ured their daily patterns of energy use using respirometry. Hummingbir ds showed considerable flexibility in their patterns of energy use. Th e amount of energy used at night was positively correlated with the su rplus 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 f requency than those fed on higher concentrations. We conclude that ene rgy maximization is probably an inappropriate assumption for birds tha t are not growing, storing fat, or reproducing. We present a modificat ion of the original model that allows assuming that birds do not maxim ize energy intake, but rather maintain constant rates of energy intake . We describe experiments and criteria that allow discriminating among the two models. (C) 1997 Elsevier Science Inc.