The feathers of hummingbirds are iridescent due to the inhomogeneous interference structure of platelets on feather barbules.
Costas Hummingbird (Male) , Visitors Center, Anza Borrego Desert State Park, Borrego Springs, California
Broad-Billed Hummingbird (Male) , Santa Rita Lodge, Madera Canyon, Near Green Valley, Arizona
Rufous Hummingbird , Reifel Migratory Bird Sanctuary, Ladner, British Columbia
“To summarize, hummingbird iridescence is due to interference colors
produced by a stack of about three films whose optical thickness is
one-half the peak wave length. Each film is a mosaic of platelets of
elliptical form. Each platelet is about 2.5 microns long and one micron
wide. The platelets are not homogeneous and consist of air bubbles
encased in a matrix of refractive index about two. The different
hummingbird colors are produced by a combination of effects. The
platelet thickness decreases moderately as one passes from red through
green to blue, and the air content increases simultaneously. In theory,
of course, platelet thickness could remain constant and the color change
would arise solely by variation in air content. Conversely air content
could be constant and the platelet thickness varied. Nature for reasons
best known to herself has elected to vary both factors together.”
(Greenewalt et al. 1960:253)
“First, we repeated the examination of barbular surfaces with the optical microscope. We confirmed the presence of the platelet mosaic on the iridescent surfaces of fifty or so hummingbird species. Only the platelets are colored; the interstices are dark. The platelets are minute, about 2.5 microns across the long axis of the ellipse, and one micron across the short axis. Ten thousand of them laid end to end would measure a little over an inch. Their size varies little throughout the hummingbird species which we have examined. The length of 2.5 microns is a good average and the limits of variation would be no more than 30 per cent either way. Along its length the barbule is divided into cells separated by diagonal lines crossing the width of the barbule. At the points where the barbule joins the ramus and where the pennulum develops, the colored platelets disappear and one sees only a few random uncolored or faintly colored ellipses in these areas. The barbule proper then has a surface which is 15-20 microns wide and 100 microns long, divided by diagonal boundary lines into a series of cells which look like parallelograms, each cell made up of a mosaic of 100 or more beautifully colored elliptical platelets” (Greenewalt et al. 1960:250).
