Feather Color Is a Costly “Complex System Design”

The brilliant, shimmering colors in the breast feathers of the Bird of Paradise have long fascinated ornithologists.  Alfred Russell Wallace was perhaps the first Englishman to see the magnificent birds in their native Malaysian habitats and wrote, “the richness of their glossy orange colouring, and the exquisite delicacy of the loosely waving feathers, were unsurpassable.”¹
    Now, with the use of electron microscopes, scientists are beginning to understand how feathers are able to flash such intense colors.  We now know that the colors are not produced by pigments but by organized geometrical patterns, called photonic crystals, giving the phenomenon “structural color” through refraction instead of pigmentation color through reflection.  According to Pete Vukusic [U of Exeter] in Current Biology,² the structure is more complex than thought:

The performance and function of such simpler systems as biological multilayers are well recognized, but despite recent progress in the use and development of measurement and modeling techniques in this area, there are many other structurally coloured systems whose detailed action and function are poorly understood.  This is largely due to the morphological complexity of their systems’ inherent photonic structures, which makes it difficult to discern their role and effectiveness.  Photonic effects arising from complex system designs are usually attributed generically to coherent scattering and their purpose ascribed generally to conspecific communication or to camouflage.  This is rather too vague, however, and it invariably overlooks strategic design features that have key implications in aspects of display behaviour.

Vukusic, an expert on insect structural color, elaborated on some of the structural complexities in photonic crystals: (1) multiple photonic systems interacting together; (2) absorbing pigments that modulate the optical effects; (3) variations on the refractive index periodicity, “a biological noise of sorts, which supplements or complements the sample’s inherent structural order or quasi-order.”  In addition, these complexities may be enveloped in larger geometrical envelopes that modulate the far-field effect.  “Achieving a fundamental grasp of any resulting light-manipulation mechanisms,” he said, is no easy task.”
    A recent study pointed out something interesting in the feathers of the Bird of Paradise feathers (Parotia lawesii): “This bird species is well-known for its ultra-bright and rapidly-changing saturated iridescent chest-feather colours that take centre-stage during courtship display,” Vukusic noted.  The feathers make use of melanin cylinders and air spaces in the keratin matrix of the barbules, probably tuning the refractive index of the colors produced by the periodic patterns.  For picture of the bird see press release on U of Groningen site.
    Even more intriguing, though, the barbules in cross section have a boomerang shape.  The new study discovered the effect: Stavenga et al “confirmed that the multilayer geometry imposed by the uniquely curved cross-sectional shape of this species’ barbules enables it to reflect intense saturated colour concurrently in three different directions.”  This shape also causes “unusually abrupt and dramatic change in feather hue with changes in the angle of observation,” such as when the male orients himself in front of the female during courtship displays.  Vukusic noted, though, that its effect on female behavior has not been established.  Nevertheless, “This is an optical effect which a human observer notices but which until now has been unquantified and its mechanism poorly understood.”  Stavenga et al said with evident delight:

This allows each barbule to work as three coloured mirrors: a yellow-orange reflector in the plane of the feather, and two symmetrically positioned bluish reflectors at respective angles of about 30°.  Movement during the parotia’s courtship displays thereby achieves much larger and more abrupt colour changes than is possible with ordinary iridescent plumage.  To our knowledge, this is the first example of multiple thin film or multi-layer reflectors incorporated in a single structure (engineered or biological).  It nicely illustrates how subtle modification of the basic feather structure can achieve novel visual effects.

The power of iridescent flashing doesn’t just happen.  It involves control at the microscopic level in three dimensions, and a fourth dimension of time in the way the male manipulates the feathers for best effect.  “Control not just of colour reflection but of the directional distribution of colour reflectance is likely to be a key behaviourally-linked characteristic of many iridescent animals and plants,” Vukusic said, pointing out that the structural color controls angle, wavelength, polarization simultaneously.  The reference to “engineered or biological” structure by Stavenga et al hints that engineers might well learn some novel visual tricks from the bird of paradise.
    Why would a bird invest such energy in these microscopic mechanisms?  “Rapid and intense changes in animals’ structural colours, whether through dynamic or orientational means,” Vukusic pointed out, “require structures or processes that are costly to create.”  He did not speculate about evolution of this capability; instead, he pointed out the design that contributes to adaptation: “Such are the costs and, invariably, such is the optical efficiency and the optimisation of the systems’ designs, that significant biological function should genuinely be served.”  And indeed it is: the “feather barbule design appears very highly adapted for the purpose of promoting during display stronger and faster hue shifts when compared to most other iridescent feather systems.”  Further investigation “will yield superior understanding of the biological function such adapted iridescence strategies offer their hosts.”

1.  Michael Flannery, Alfred Russell Wallace: A Rediscovered Life (Discovery Institute Press, 2011), pp. 34, 51.
2.  Pete Vukusic, “Structural Colour: Elusive Iridescence Strategies Brought to Light,” Current Biology, Volume 21, Issue 5, R187-R189, 8 March 2011, 10.1016/j.cub.2011.01.049.
3.  Stavenga, D.G., Leertouwer, H.L., Marshall, N.J., and Osorio, D., “Dramatic colour changes in a bird of paradise caused by uniquely structured breast feather barbules,” Proceedings of the Royal Society of London B (2010), epub ahead of print.  From reference provided by Vukusic; see abstract at Pub Med.

This fascinating article was a good example of how to write science without Darwin.  Vukusic did not mention evolution one time.  Maybe he just forgot, but who needed a Charlie story tacked onto the observational facts anyway?  This was good old-fashioned scientific investigation.  The researchers noticed a trait that was intriguing and beautiful, and wondered how it works.  They used advanced microscopes to investigate the structure.  They determined which geometric features produce the optical effects.  They observed that the trait is costly yet well adapted to the birds’ mating behaviors, noting that the “feather barbule design appears very highly adapted for the purpose” of sexual display, to say nothing of the fascination they arouse in human observers.
    The lack of Darwin-talk was intensified by design language in the article.  Though an evolutionist himself (see quotes in New York Times article), Vukusic in this paper spoke only of feather design, adaptation, and purpose.  Everybody can accept that.  This illustrates how intelligent design concepts in an article do not require any religious references.  The reader can make those inferences himself.  “Just the facts, ma’am” is all that is required in a scientific paper to point where the evidence leads, without taking the reader there by the hand.  We applaud Dr. Vukusic for not turning on the Darwin fogma machine.  Nobody wants to hear that butterflies and birds arrived at these superior designs by “convergent evolution” or how natural selection paid the high cost of photonic crystals just to pass on bird genes.
    By analogy, the bird of paradise feather illustrates how to write a scientific paper fit for paradise.  The structural color is produced by facts ordered to produce a design inference.  Make it as bright and shimmering as possible, from every angle.  The melanosome cylinders can act as absorbers for Darwinian wavelengths that serve no purpose other than to interfere.  The effect on the reader will be intense admiration and appreciation for brilliant light.