These
two images are included to show the effect/importance of light on
showing the iridescent colours on the throat of the hummingbirds, in
this case the Green-throated Carib. They are of the same bird, taken
seconds apart, with the only difference being that the bird has turned
its head, thereby changing the angle at which the sunlight strikes the
feathers in the neck. This area is known as the gorget.
The following is a scientific explanation as to colour and iridescence
seen in the feathers of Hummingbirds. It is taken from Paul A.
Johnsgard's book, "The Hummingbirds of North America" (1997, second
edition, published by Smithsonian Institution Press in Washington, DC.)
The highly iridescent feathers of the hummingbird gorgets are among the
most specialized of all bird feathers. But even in the male's gorget
... only about the distal third of each feather is modified for
iridescence; the close overlapping of adjacent feathers thus generates
the unbroken color effect. The iridescence is produced by the proximal
part of the barbules, which are smooth, flattened and lack hook-like
barbicels or hamuli. Beyond the color-producing portion, the barbule is
strongly narrowed and curved toward the distal tip of the feather. The
barbicels in this area help to hold together the barbules on the side of
the barb, but do not unite the barbules of adjacent barbs.
(Aldrich,1956).
...The colors do not directly depend on
selective pigment absorption and reflection, as do brown and blacks
produced by the melanin pigments of non-iridescent feathers. Rather,
they depend on interference coloration, such as that resulting from the
colors seen in an oil film or soap-bubble. Basically, the colors depend
on light being passed through a substance with a different refractive
index than air (1.0), and being partially reflected back again at a
second interface. The percentage of light that is reflected back
increases with the difference in the refractive indices of the two
media; in addition, the thickness of the film through which the light is
passed strongly influences the wavelengths of light that are reflected
back. Put simply, red wavelengths are longer than those at the violet
end of the spectrum and generally require films that are thicker or have
higher refractive indices than those able to refract bluish or violet
light. Thus, the optimum refractive index for red feathers is about
1.85; for blue feathers it is about 1.5.
Hummingbird feathers
may attain any refractive index within this range because the
iridescence portions of the barbules are densely packed with tiny,
tightly packed layers of platlets. These platlets are only about 2.5
microns in length and average about 0.18 microns in thickness, but they
vary in thickness and are differentially filled with air bubbles. The
platlets matrix, probably of melanin, evidently has a refractive index
of about 2.2, whereas the air bubbles inside have a refractive index of
1.0. Varying the amount of air in the platelets provides a composite
refractive index that ranges from the red end of the spectrum (1.85) to
the blue (1.5)....
Thus, the actual thickness of the platlets
not only significantly determines the quality of the perceived light,
but it also affects the amount of air held within the pigment granules
and the consequent variations in interference effects. Further, a single
pigment granule can produce different color effects according to the
angle at which it is viewed. When an optical film is viewed from about,
it reflects longer wavelengths than when viewed from angles
progressively farther away from the perpendicular. Thus, a gorget may
appear ruby red when seen with a beam of light coming from directly
behind the eye, but as the angle is changed the gorget color will shift
from red to blue and finally to black, as the angle of incidence
increases (Greenwalt, 1960a).
In hummingbirds, the
color-producing pigment platlets are closely packed into a mosaic
surface, and 8 to 10 such layers are then tightly stacked on top of one
another in typical iridescent feathers. Far from confusing the visual
effects, such stacking actually tends to intensify and purify the
resulting spectral color, which is why hummingbirds have possibly the
most intensively iridescent feathers known in birds (Greenwalt, 1960a).
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