The research, led by the Monash University School of Biological Science Associate Professor Martin Burd, and published in the Proceedings of the Royal Society B, used mathematical models of bird vision to represent the colours of Australian flowers as birds themselves perceive colours.
Professor Burd, together with a team of biologists from Monash and RMIT universities in Melbourne and Bucknell University in the US, then used new statistical techniques to show that a small range of novel colors evolved among bird-pollinated flowers far more often than would occur by chance.
In the study, Professor Burd and his colleagues measured the spectral reflectance of flowers (the wavelengths of light reflected back from petals) of 234 native Australian species of flowers, consisting of 80 bird-pollinated species and 154 insect-pollinated species.
“In our study, we measured these profiles across the ultraviolet and visual wavelengths of light,” Professor Burd said.
“We humans have three types of colour receptors in our retinas, sensitive to blue, green, and red light. However, birds have four types of receptors, so what they see is far more complex than what we see. Even among birds there is variation. Some birds, including Australian honeyeaters, have violet-sensitive receptors along with blue, green and red. Other birds use receptors sensitive to ultraviolet rather than violet light.”
The researchers translated the spectral reflectance patterns of flowers into single points in a ‘colour space’ that represents the sensitivities of the four kinds of colour receptors in bird vision.
According to the research, about half the flowers that were bird-pollinated overlapped with the colour space occupied by insect-pollinated flowers. Professor Burd claims the similarity in colours was not surprising, since bird pollination usually evolved from insect-pollinated ancestors.
“The other half of bird-pollinated secies, however, were crowded in a narrow and separate stretch that occupied just one per cent of the total volume of the colour space. These colours appear red to humans, and we named this cluster the ‘red arm’. Their reflectance profiles would also strongly stimulate the red receptors in bird eyes.”
Professor Burd said the research indicated that many flowering species had evolved to ‘talk’ to birds using a very particular set of colour ‘words’.
“In particular, the disctinctiveness of the ‘red arm’ appeared only when the colour space was based on the visual system of honeyeaters.
“Models of the other visual system in birds with ultraviolet receptors, common in parrots and songbirds, did not reveal a distinctive red arm of floral colours.”
Professor Burd said this was not surprising given honeyeaters were the main bird pollinators in Australia. The research team concluded that there has been significant evolutionary convergence in the colour of bird-pollinated flowers in Australia, and that there visual system of honeyeaters has been the apparent driver of this convergence.
“The birds are talking honeyeater language because that’s where all the action is!”
Professor Burd is now hoping to investigate whether the same evolutionary convergence that shaped the red arm among Australian floral colours also occurred appear among American plants visited by hummingbirds, which, like honeyeaters, have a visual system based violet sensitive photoreceptors. Dr Burd argues that in contrast, Asian and African flowers visited by songbirds, which use ultraviolet-sensitive receptors should not have significant convergence on the red arm.
“Is this the case? It’s a question we hope to answer next.”