Page 82 - The Origin of Birds and Flight

Stuart Burgess, an assistant professor of engineering design at Bristol
University in England, revealed the extraordinary nature of the peacock feather
in a most striking way and concluded that this structure could not be explained
in terms of Darwin’s theory of “sexual selection.”
With its large tail feathers with their vivid hues and unique patterns, the pea-
cock possesses an extraordinary beauty. One feature of these stunning irides-
cent colors is that they change according to the angle one views them from.
These are created not thanks to pigments (the substance that gives color to
feathers ), but to an optical effect in the micro-hairs known as the “thin film.”
The thin film effect that appears in the micro-hairs—the smallest structures
in bird feathers that can be seen only under a microscope—takes place in three
keratin layers.
The transparent keratin layers refract light, while retaining some compo-
nents of that light. The fact that the soft inner part is brown prevents light pass-
ing through and disappearing by providing a dark background. The reflected
light can thus give rise to colors. The thin film effect takes place in the three lay-
ers at the same time, producing different shades. It is possible for the keratin
layers to produce a specific color only thanks to their being very thin, just
1/20,000 of a millimeter (0.000,001,969 of an inch), the ideal thickness for pro-
duction of the most bright colors. The thickness of each layer must not exceed
the wavelength of visible light. The extraordinary vividness of the eye patterns
in the peacock's tail stems from this unique color-production mechanism. 1
The eye shape at the end of each feather emerges through a combination
of thousands of micro-hairs. Although they are independent of one another,
thousands of neighboring micro-hairs produce this pattern. Were they arranged
at random and in an unordered manner, they could not produce the geometric
shapes based on mathematical formulae not detailed here. The odds of this
shape arising by chance are as slim as that of the flowers in a garden combin-
ing to produce the same pattern.
During courtship, an adult male peacock produces a magnificent fan as it
displays its tail feathers. An adult peacock’s approximately 200 tail feathers are
replaced each year. Around 170 of these are the ones forming the pattern of an
eye, while the rest form a T shape. At the microscopic level, the T and eye
feathers can be seen to have a very complex structure. Each of the eyes can
be seen, because the short feathers have been located toward the front of the
tail and the longer feathers at the back.