Hummingbird foraging at experimental patches of flowers: evidence for weak risk-aversion
Abstract
Hummingbirds foraging for nectar in mountains of the western USA encounter spatially-variable rewards under energetically-demanding environmental conditions. Previous workers have found that hummingbirds respond quickly to the average quantity and quality of reward; in addition, one might expect a response to reward variance. To explore this possibility we observed free-flying hummingbirds visiting two arrays of the native plant Ipomopsis aggregata. All flowers in one array received a 1 lI aliquot of artificial nectar whereas every fifth flower in the other array received 5 stl. This yielded a mean nectar standing crop similar to that found in nature, and variances in standing crop that were only slightly more extreme than those in nature. Over 12 days we recorded 362 foraging bouts by male and female Broad-tailed Selasphorus platycercus and Rufous S. rufus Hummingbirds. The high-variance (HV) array received 8.9% fewer foraging bouts than the low-variance (LV) array, and this difference was significant statistically at the ao = 5.4% level. Bout frequency varied among periods of the day, but not differentially by nectar treatment. Foraging bouts tended to be longer in the LV array, though not significantly so, and hummingbirds visiting both arrays in sequence were significantly more likely to visit the LV array first. These results suggest weak risk-aversion under natural conditions. Lack of a stronger response may derive from the fact that hummingbirds sample thousands of flowers a day, so that the total profit realized over a day is not strongly influenced by flower-to-flower variation.
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