Anton Pauw spent 17 days staking out three remote sites along the west coast of South Africa. His mission: To find out how a certain milkweed flower, Microloma sagittatum, was pollinated. All other milkweeds are pollinated by insects, and Pauw, a graduate student in botany at the University of Cape Town, wondered what insect species visited M. sagittatum. To his surprise, he found that it wasn't an insect that pollinated the milkweed but a bird. Moreover, the bird carried the pollen from flower to flower in a most unusual way: on its tongue. This is the first known case, says Pauw, of pollen being transferred on the tongues of birds.
Other birds, and even bats, are known to pollinate flowers. But in those cases the pollen sticks to feathers or fur. In the lesser double-collared sunbird that Pauw observed, the tip of the bird's tongue was often covered with pollen.
M. sagittatum has many of the characteristics of a bird-pollinated flower, says Pauw. It is red--a color attractive to birds--secretes plentiful nectar, and is unscented. The flower's pollen is stored in five removable parcels; each contains two pollen-filled sacs with a small clip on the end. In other milkweed species, this hook attaches to the bristles or appendages of insects. But M. sagittatum's pollen sacs hook onto the odd forked tongue of the sunbird.
When the sunbird visits the milkweed, it probes the flower with its bill and uses its tongue, rolled up lengthwise along the sides like a straw, to sip nectar. Pollen sticks to the tip of the forked tongue. Pauw suspects that other birds, such as hummingbirds and honeyeaters, may also pollinate by tongue; like sunbirds, their tongues are adapted to drinking nectar. These birds, along with the sunbird, were thought to be "robbing" nectar from the flowers without pollinating them. "But when I caught the sunbird," says Pauw, "I could actually see pollen on the tip of its tongue."