Hummingbirds' nectar concentration preferences at low volume: The importance of time scale

  • Roberts W
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Abstract

Although average nectar sugar concentrations in hummingbird-pollinated plants are low (20-25% sucrose-equivalent sugars; mass/total mass), in choice tests birds have preferred concentrations higher than 45%. This discrepancy might be explained by the use of large nectar volumes in all published choice tests to date. Biophysical models predict that low concentrations maximize energy intake rates, and thus may be preferred, at small volumes characteristics of the flowers that birds visit. Rufous hummingbirds', Selasphorus rufus, concentration preferences were tested and their intake rates measured at low volume with a computer-controlled food delivery and activity-monitoring system. Contrary to the predictions of models, birds generally preferred 65% sucrose over more dilute solutions. This preference generated near maximal energy intake rates over the time scale of foraging bouts, even though rates over flower visits and shorter time scales suggested that lower concentrations would be more profitable. Neither hummingbirds' preferences nor the energetics of nectar extraction in this low-volume presentation account for the low nectar sugar concentrations characteristic of the flowers they pollinate.

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Authors

  • W. Mark Roberts

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