The ruby-throated hummingbird has to be one of the most eagerly awaited spring arrivals for Maine birders. Their small size, aerial antics and general feistiness are all endearing qualities. And how stunning the iridescent red on the throat patch, the gorget, is.

I’ve been contacted several times this summer by people who describe a hummingbird with a black throat. Although some hummers do have black gorgets, none regularly occur in eastern North America. Why would the gorget of a ruby-throated hummingbird appear to be black?

Physics explains how the gorget feathers of a hummingbird show that sparkling iridescence. Hummingbird feathers have black melanin granules that function as color pigments. In most birds, the melanin granules are randomly scattered in the feather structure.

In hummingbirds, the melanin granules are hollow and flattened like pancakes. Furthermore, they are stacked neatly in seven to 15 rows.

When light enters a gorget feather, some colors are absorbed by the black melanin granules and others (red in the case of our ruby-throated hummingbird) are scattered back out to an observer’s eye.

The angle at which the light passes through the stacks of melanin granules changes the distance photons of light will move. Consider a cube. If light enters exactly perpendicular to the cube, it will travel a minimum distance to the other side. But if light enters at an angle, it will have to travel a greater distance to pass through the cube.

The changing angles alter the red color that is reflected back. So, when a hummingbird turns its head, the gorget seems to change from one shade of red to another in a shimmering fashion. That is how iridescence works in hummingbirds.

If the sunlight hits the gorget at a particularly flat angle, the color that is reflected is dark, almost black. When the hummer turns to a more favorable angle with the sun, the red pops out.

The shape and size of the melanin granules determine the color that is reflected iridescently. The blue-throated hummingbird found in Arizona and points south has granules that produce a breath-taking blue iridescence.

Iridescence is not restricted to hummingbirds. Shimmering colors are produced in birds of paradise, trogons and some starlings.

In many birds, melanin pigments produce feathers that are black, brown or tan. Carotenoid pigments produce the red feathers of a cardinal, the orange of an oriole and yellow of a goldfinch.

Iridescence is said to be a product of structural coloration. Feather pigments do not produce the color but rather interfere with light to reflect a particular portion of the visible spectrum.

No bird has blue pigment. The blue of a blue jay or eastern bluebird is produced by structural coloration. Photons of white light enter a feather and only the blue wavelengths are reflected back out.  Jays and bluebirds do not have their melanin granules arranged in tiers as in hummingbirds so they do not show iridescence. The blue color is always the same.

If you take a blue feather and hold it up to the light, it will appear gray because the blue light is scattered on the opposite side of the feather but not to your retinas.

If a bird combines a feather that produces blue structural color with yellow carotenoid pigment, the feather will be green. With a few exceptions, birds cannot produce green feathers using a pigment.

Structural coloration is also used in bird skin and eyes. Blue or green skin produced by structural coloration is known in at least 50 families of birds. Arrays of collagen fibers rather than melanin granules do the trick here. (Collagen is a common protein providing structural support in tendons, ligaments and skin of birds, mammals and other animals.)

In similar fashion, chemical pigments interact with structures in the eye to produce the often striking iris color in birds.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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