Why are Hummingbirds Iridescent?

Hummingbirds have brilliant color; their feathers have a metallic sheen that is remarkable and unique.

The following text is from Paul A. Johnsgard's book, The Hummingbirds of North America (1997, second edition, published by Smithsonian Institution Press in Washington, DC.)
It is used with the permission of the publisher and the author.

 

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 brbill-s.gif (2771 bytes)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 producedbrdtail.jpg (18807 bytes) 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 wavelengthscallip-s3.gif (4059 bytes) 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 brbill-s.gif (2771 bytes)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 platlets 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,anna-sx.gif (2345 bytes) 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 intoanna-sx.gif (2345 bytes) a mosaic surface, and 8 tobrbill-s.gif (2771 bytes)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).

Johnsgard's sources:

Greenwalt, C.H.(1960), Hummingbirds. Doubleday and the American Museum of Natural History, Garden City.

Aldrich, E.C. (1945). Pterylography and molt of the Allen Hummingbird, Condor 47:137-148.

 

 

 

NOTE: 
  • The gorget is the brilliantly colored area on the throat of a male hummingbird.
  • Distal means outward part and proximal means inward part.
  • A feather consists of many tiny sub-feathers, called barbs.
  • The barbs consist of many tiny sub-feathers called barbules. Barbicels are tiny hooks that hold the barbules together.
  • Back to article.                              - L.G.

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