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Feathers Reveal Their True Colours
kathryn{at}biologists.com
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Birds exploit a wide variety of pigments to produce dramatic effects, and their vivid plumage has always fascinated man. By using the reflective properties of the feather's materials, birds have extended the range of visual effects by creating `structural colours' that produce shimmering iridescent effects. These structural colour effects are produced by reflection, in a similar way to the brightly coloured rings in oily patches. Daniel Osorio realised that although a great deal was known about the pigmented colouring, no one had a systematic understanding of the reflected spectral properties of feathers. If he was going to get any idea of how birds exploited their extended pallet of colours, he had to develop some way of systematically analysing a feather's reflected spectra. Together with graduate student Abi Ham, they built an optical system where they control the feather's orientation under a moving light source to collect the definitive spectral signatures (p. 2017).
Each feather's barb is made up of microscopic layers of melanin, keratin and air. Light reflects off the boundaries between each layer, so that some wavelengths become stronger while others vanish, giving each feather a characteristic spectrum. Most feather's colours are produced by a combination of pigmentation and reflected optical properties, but some feather's colours are produced by the structural colour effect alone.
Osorio and Ham constructed a system where they could record the intensity of each reflected wavelength as a light source moved over a feather. Through a collaboration with another team at Sussex, they had access to plumage from 15 different species ranging from the dowdy pigeon to the brilliant kingfisher. Ham and Osoria chose five types of feathers to investigate the range of visual effects produced by structural colouration. By collecting a diffuse spectrum that was reflected off a single point and then slowly rotating it to see how the spectrum varied at different angles, the two scientists collected the complete spectral features for each feather.
Comparing the range of frequencies that produced each feather's colours, Osario and Ham realised that some feathers reflected a narrow band of wavelengths, while others reflected a wider slice of the spectrum. The other remarkable feature of the feathers was the directionality of the reflected colour. Hummingbird feathers looked completely black from some directions, while the feather's vivid colour only appeared at certain discrete angles. This surprised Osorio, because it meant that the microscopic mirror structures that reflect specific colours aren't always aligned with the feather's surface.
Having established that his new approach to measuring a feather's colour is extremely effective, Osorio is beginning to look beyond individual feathers and at the net effect of iridescence on an individual's plumage. It isn't clear whether there is real information in a bird's reflective display or the reflected colours are simply a cheap way of getting attention, but Osorio's method for collecting spectral data offers the first systematic approach to seeing feathers in their true light.
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