When hummingbirds forage for nectar in the mountains of Colorado, they avoid flowers that have already been raided by nectar-robbing bumblebees. They do this without having to see the nectar, or remember which plants they have visited before, according to postdoctoral researcher Rebecca Irwin at the University of California, Davis.
Broad-tailed and rufous hummingbirds feed on nectar from the long red flowers of scarlet gilia (Ipomopsis aggregata). The birds play an important role in pollinating the plants. But bumblebees also have a taste for nectar. The bees chew holes in the base of the flower to get at the nectar, and do not help pollinate the plants. Nectar-robbed plants are less likely to get pollinated and produce up to 50 percent less seed, said Irwin.
Irwin simulated nectar robbing by artificially removing or adding nectar to flowers, then watched hummingbirds feeding on them. The birds were less likely to visit a plant without nectar, and probed fewer flowers on plants they did visit.
Two possible explanations are that the birds use visual clues that tell them the flower is empty, or that they remember robbed flowers from previous visits, she said.
To block visual clues, she painted over the flower bases to disguise bee holes and nectar content. Despite this, the birds still went for the full flowers.
To test the bird's spatial memory, Irwin set up full and empty flowers in an aviary and let the birds get used to them. Then she moved the flowers around. The birds still went to the full flowers, without wasting energy on empty ones.
Irwin has some more theories to test on why the birds pick the right flowers. One is that their wingbeats make the flowers vibrate, and full flowers have a different frequency to empty ones. She will be testing these ideas in next year's research season at the Rocky Mountains Research Laboratory, Colorado.
EDITOR'S NOTE: Color photographs of hummingbirds, Ipomopsis flowers and bumblebees are available. Contact Andy Fell for details.