Sunday, 9 July 2006

Blue in the face

Honeyeaters are an important element of the bird fauna in Australia, New Guinea, New Zealand and parts of the Pacific, including the Hawaiian islands.

The blue-faced honeyeater (Entomyzon cyanotis) of northern and eastern Australia is one of the larger species. As the name indicates, it is characterised by the blue mask around the eyes in adults. (What? You didn't notice?) Blue is a structural colour. It isn't produced by pigments but by the scattering of incident light. Until recently, structural blue was attributed to Rayleigh scattering—reflection of light from random particles. But detailed studies revealed there's nothing random about the way in which light is scattered in bird skin. Collagen fibres are arranged in a regular array to produce the colour. The fibre pattern determines the hue.

Why the blue mask? Is it for signalling to others? Is it a gauge of health for prospective mates? Maybe both or neither of the above. But many homeyeaters (as well as other birds) have coloured facial skin.

Blue-faced honeyeaters were formerly placed with the miners (Manorina) and wattlebirds ( Anthochaera) on the basic of size and presence of coloured skin. DNA analysis showed that they had nothing to do with the miners but were more closely related to the smaller Melithreptus honeyeaters. File it under B for the bleedin' obvious. (Hindsight is a wonderful thing.) The plumage of the two genera is almost identical—black head with white nape, olive green back and wings and white belly. The only difference is the larger size (pfft!) of the blue-faced honeyeater and the extent of the coloured skin around the eyes. In Melithreptus, the skin is restricted to a half moon above the eye. The colour varies between species, ranging from pale blue to bright red.

And that brings us to another question about honeyeaters and colour ... Blue is a structural colour but red is a pigment. Within the one genus we have a variety of coloration with widely different origins. Which came first? The cerulean or the red?

Read more

Driskella, A.C. & Christidis, L. (2004). Phylogeny and evolution of the Australo-Papuan honeyeaters (Passeriformes, Meliphagidae). Molecular Phylogenetics and Evolution 31: 943–960

Prum, R.O & Torres, R.H. (2003). Structural coloration of avian skin: Convergent evolution of coherently scattering dermal collagen arrays. Journal of Experimental Biology 206: 2409–2429.

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