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Hummingbirds’ concentration preferences and the energetics of nectar feeding : predictions, tests and implications for optimal foraging theory and pollination biology Roberts, William Mark


An important issue in pollination biology and foraging theory is that average nectar sugar concentrations in hummingbird-pollinated plants are less than half those preferred by birds in published choice tests. One current explanation for this discrepancy is that low concentrations maximize birds’ energy intake rates. Previous workers have suggested that birds may prefer low concentrations at the nectar pool volumes characteristic of the flowers they visit, which are much lower than volumes used in all previous choice tests. I used three approaches to study this issue. I modelled hummingbird visits to flowers on three temporal scales: tongue loading, the licking cycle, and entire visits to flowers. The nectar concentration that maximizes energy intake rate increases with the temporal scale of integration, so that optimal nectar concentration for the licking cycle is higher than predicted by models that integrate over only the loading phase of single licks. Since birds must position, insert, and withdraw their bills in addition to licking nectar, the optimum at the scale of flower visits is even higher. This "overhead time cost" of handling flower morphology, for most non-traplining hummingbirds under most natural conditions, is as great as or greater than the cost of handling nectar. My modelling suggests that for these birds, the potential variation in the fine-scale factors that determine nectar intake rate during licking has little effect on flower handling time, and therefore is unlikely to determine optimal nectar concentration or the profitability of visiting flowers. To test the models' validity, I measured fine-scale parameters of hummingbird licking with a photodetector array that monitored movement of the tongue and nectar pool meniscus. The results allowed me to reject previous hypotheses about the details of licking, but they supported the qualitative model prediction that optimal nectar concentration is low at the time scale of licking. To determine whether hummingbirds prefer concentrations that maximize energy intake rates over the licking cycle, over feeder visits or over foraging bouts, I tested concentration preferences at low nectar pool volume with a computer-controlled food delivery and activity-monitoring system. Birds preferred the highest concentration provided, which maximized energy intake rates over foraging bouts but not over finer time scales. The low nectar sugar concentrations characteristic of flowers pollinated by hummingbirds are not accounted for by birds’ preferences nor by the energetics of nectar extraction.

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