THE UNDERSTORY: Hummingbirds and Their Friends

By Susan Starr.

Hummingbirds have found my garden. I have to confess that it wasn’t intentional. When I introduced more drought-tolerant plants to my landscape last year, including Cuphea and Grevillea, it didn’t take long for the hummingbirds to arrive. They seem particularly to enjoy the orange and red Cupheas and the Grevillea ‘Long John’.

I could understand why Cuphea, which are native to the tropical and temperate regions of the Americas, might have evolved to be attractive to San Diego hummingbird pollinators. However, the Grevillea's attractiveness puzzled me. Like Cuphea, my Grevillea ‘Long John’ is red and has clusters of tubular blooms, both of which are hummingbird magnets. However, there are no hummingbirds in Australia, so they cannot rely on hummers for pollination. How, I wondered, are Grevillea pollinated in their native environment?

It turns out that there are at least three species of nectar-eating birds that feed on plants with tubular flowers: hummingbirds in the Americas, sunbirds in the Old World, and honeyeaters in Australia. In an example of what scientists call convergent evolution, all three of these species have similar morphologies, reflecting their similar lifestyles. However, they are not at all closely related. Hummingbirds are related to swifts, sunbirds to crows, and the honeyeaters belong to yet another family, the Meliphagidae.

All these birds have beaks and tongues designed to allow them to extract nectar from plants at similar rates, using capillary action in their tongues. Honeyeaters have curved beaks and a highly specialized, extendable, brush-tipped tongue with a pointed tip; sunbirds have long curved-downward beaks and brush-tipped tube-shaped tongues; and hummingbirds typically have straight or downward curving beaks and tongues with hairs at the end to help them sip nectar.

These are generally small birds; an Anna’s hummingbird weighs around 4 grams, similar to the larger sunbirds, while honeyeaters are somewhat larger. They can all hover, but hummingbirds do so much more consistently than sunbirds and honeyeaters, both of which prefer to perch. Hummingbirds and sunbirds are brightly colored; honeyeaters tend to have duller plumage. So in Australia, Grevillea would be pollinated by honeyeaters, while in Southern California, it’s the hummingbird who does the job.

A word of caution. Although sunbirds, hummingbirds and honeyeaters may have evolved to look alike, that does not necessarily mean they are interchangeable pollinators. Convergent evolution produces plants and animals that are similar, but not identical. Sometimes that matters. The Bird of Paradise, Strelitzia reginae, is a classic example of convergent evolution falling short. A sunbird pollinates Bird of Paradise in its native South Africa; when the sunbird perches on the flower, the petals open up from the weight of the bird, and pollen coats the bird’s breast and feet. In the Americas, hummingbirds instead of sunbirds sip nectar from Strelitzia, but as hummingbirds hover rather than perch, no petals open and no pollen is spread. Most often, Bird of Paradise must be hand pollinated in the Americas.

Susan Starr is SDHS Newsletter editor and a former science librarian.

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