Australian flowers bloom red because of honeyeaters

Australia is a bird lover’s delight. Even those of us who haven’t taken up birding as an obsession can’t help but be delighted by the wealth of brightly colored, charismatic species.

One large and diverse group of birds commonly found in Australia is the honeyeaters. These small- to medium-sized birds feed on the nectar of flowers, similar to the hummingbirds of the Americas (though honeyeaters don’t share the hummingbirds’ ability to hover).

Flowering plants don’t feed honeyeaters — or any other kind of pollinators — as a public service. The nectar serves as a lure, and the birds pick up pollen from one flower or plant and take it to another, which enables fertilization and reproduction. Many Australian flowering plants have thus converged on a similar method of drawing honeyeaters to them — producing flowers in colors that stand out to the birds, Martin Burd of Monash University in Melbourne, Australia and colleagues report February 25 in the Proceedings of the Royal Society B.

The researchers looked at 234 flowering plants native to Australia — 155 pollinated by insects, 57 pollinated by birds and 22 by both — and used a computer algorithm to convert the variety of wavelengths reflected by a flower into a single value that could be compared to the color vision system of a bird pollinator. Some birds have vision systems similar to humans, with receptors for blue, green and red wavelengths. Another group that includes parrots also has a receptor for ultraviolet wavelengths. A third group — this one contains the honeyeaters — has a receptor for wavelengths in the violet range.

The colors of flowers pollinated only by birds weren’t all that different from those visited only by insects, the researchers found. But about half of the bird-pollinated blooms fell into a similar color space that humans see as a shade of red and that stand out even more in the honeyeaters’ vision.

The plants aren’t all closely related; they all converged on a similar coloring for their flowers as they evolved to attract their honeyeater pollinators, the researchers conclude.

A similar convergence may exist in the Americas, Burd and colleagues write, because hummingbirds see in a similar range to honeyeaters. In Africa and South Asia, though, the dominant bird pollinators are sunbirds, which fall into the vision group that includes the parrots. So blooms in those parts of the world may have converged on a different coloring.