Some males put a lot of effort into getting their gal. Peacocks invest in stunning tails while yellow wagtails brag about their fitness with costly vivid plumage. But what messages do male mallards send to dowdy mates with their iridescent feathers? Chad Eliason from the University of Akron, USA, explains that brash displays are costly and broadcast a male's quality. ‘If there are no costs of the colour then it can't be a way to show that better quality males can produce brighter colours,’ says Eliason. He explains that these shiny colour effects are generated by flat structures in the feather in the same way that the colours on soap bubble surfaces are produced. However, these so-called structural colours are probably cheap to produce, so what costs do they impose to allow a male to show off his prowess? Knowing that the flat barbule structures that produce iridescence might make the feathers more vulnerable to wetting, Eliason and his principle investigator Matthew Shawkey wondered whether iridescent feathers come with a hidden cost: increased maintenance (p. 2157).
‘Most species of duck have a bright colourful iridescent region on their wing in males and females,’ explains Eliason, so the duo decided to find out how water repellent this iridescent patch is relative to dowdier barbs on the same flight feather. Having removed the feather's naturally occurring oil with ethanol, Eliason gently placed a 10 μl droplet of water on the dull portion of the feather, photographed it and measured the contact angle between the bottom of the drop and the feather. The droplet was almost perfectly spherical: the brown barbules repelled water well as the duo had expected. However, when Eliason placed another droplet on the feather's iridescent violet barbs, the droplet spread and the contact angle dropped from 145 deg to 109 deg. The flat iridescent barbules were less hydrophobic, but how much of an effect would the feather's structure and colour have on its ability to repel water?
Looking at the structure of iridescent feather patches with a microscope, measuring their hydrophobicity with water droplets and measuring the reflected colour, Eliason and Shawkey found that the feathers with the flattest barbules produced the deepest violet tones and were the least hydrophobic. So, the most vividly coloured feathers were the least water repellent, but at what cost?
According to Eliason, some water repellent surfaces are able to self-clean. ‘Water droplets roll off the surface and carry the dirt with them,’ he says. So, if the iridescent feathers were less water repellent would they be more difficult to keep clean?
Eliason dusted dull and iridescent feathers with microscopic silica particles, produced a gently falling mist of droplets from a hand spray, held the feathers at an angle and waited to see if the droplets could wash the feathers clean. Not surprisingly, the dull feathers emerged relatively well: the mist successfully removed 50–80% of the silica particles. However, the iridescent portion of the feathers did not self-clean, retaining up to 90% of the dust. They would require significant preening to remain in tip-top condition.
So iridescent feathers probably impose a maintenance cost on males that opt for the deepest, showiest colours, and Eliason and Shawkey are keen to look at the influence of environmental factors on the evolution of iridescent colours and the impact that their costs may have on their location in a bird's plumage.