Life

Dogs might be resistant, but it turns out you can teach an old bird new tricks. An African bird has learned to hover so that it can collect nectar from flowers, just as hummingbirds do in the Americas. The bird has an unlikely trainer: an invasive South American plant that has made its way to South Africa.

Tree tobacco produces yellow, tubular flowers and like other plants with flowers of this shape, it depends on nectar-sipping birds for pollination. In its native South America it is pollinated by hummingbirds, which have evolved the highest metabolism of any animal in order to generate enough energy to hover over flowers for long enough to drink their nectar.

There are famously no hummingbirds outside the Americas, something which has puzzled evolutionary biologists. Native Old World plants with tubular flowers usually produce some sort of perch to allow birds to sip their nectar. So when Sjirk Geerts of Stellenbosch University in Matieland, South Africa, noticed native malachite sunbirds hovering around tree tobacco flowers in north-eastern South Africa, he decided to investigate.

Winter boost

The birds were known to hover occasionally before, says Geerts, but he has discovered that some sunbirds are now getting most of their winter food from tobacco tree flowers. "This is the first time we have observed them making a lifestyle of it," he says. The sunbirds used to migrate out of the region in winter because there was no nectar. Now they stay put.

Geerts doesn't yet know what impact this is having on sunbird-pollinated plants elsewhere, or on sunbird numbers. But one thing is for sure: the tobacco plant is benefiting. By putting netting over some tobacco trees Geerts found that plants pollinated by sunbirds set three times as much seed.

The discovery casts a new light on why hover-feeding evolved in birds in the Americas but not elsewhere. Geerts speculates that at some point in the evolutionary history of the Americas, there may have been many nectar-eating bird species competing for flowers.

This would have put considerable pressure on birds to be better at hovering. In contrast, if there were fewer pollinators in the Old World, all the pressure would be on plants to evolve ways to attract them, resulting in some with elaborate perches.

Geerts now wants to find out if sunbirds keep hovering during the summer, when they are expending most of their energy on raising young.

Journal reference: Oikos (vol 118, p 573)

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