Tarro Off-leash Area

After two months of almost daily rain all of the parks are waterlogged, not so great for dog walking (or at least for the dog-walker, the dogs enjoy the chance to get muddy) but the ibis and egret are taking advantage of the new food source.

The lone white ibis amongst the flock in flight did have a good number of companions on the ground, however I concentrated my photographic efforts on trying to catch the straw-necked ibis just as the sun picked up the iridescence in their feathers. A slight shift in angle and the birds were a uniform dull black.

Iridescent colours are found in a broad range of species from tiny marine animals to insects and birds. The flashing gorgets of the hummingbirds I was trying to capture in Orange County, and the brilliant sheen of the hibiscus beetles that fascinate me when they arrive each year en masse, are examples of iridescence.

Iridescence appears to play an important role in communication between animals of the same species, especially in sexual selection. A growing number of studies suggest that iridescence plays an important role in mate choice. But iridescent colours may also help animals avoid predation, providing camouflage by breaking up the bodies outline, or producing bright flashes of colour that might briefly startle a potential predator. Iridescent colours may also serve to warn predators of the unpalatability of potential prey items. Iridescence may also have non-communicative functions such as enhancing vision, repelling water or strengthening tissue connections.

With the exception of bioluminescence, all animal colours are produced by one of two primary mechanisms, or by a combination of these mechanisms: (i) pigmentary coloration and (ii) structural coloration. Pigmentary colours are, as you might guess, produced by pigments that absorb certain wavelengths of light. Structural colours are produced by the physical interaction between light and the variations in the tissues of some animals, which lead to light being refracted in different ways. Structural colouration can be produced by interference, diffraction or scattering of the light.

Iridescent colours are by definition highly directional. Changes in viewing position can dramatically alter the appearance of iridescent colours, producing considerable changes in hue, intensity or both.

Iridescence: a functional perspective Stéphanie M Doucet1, and Melissa G Meadows J. R. Soc. Interface 6 April 2009 vol. 6 no. Suppl 2 S115-S132