Earth on the Wing: May 2012

Monday, May 21, 2012

Are blue birds really blue?

Swallow Tanager, near Maquipucuna Reserve, Ecuador

I saw a Great Blue Heron fly over the Blue River today. The Blue River is not blue in color, and it made me wonder, are blue birds really blue?

Great Blue Heron, Cheyenne Bottoms Wildlife Area, Kansas

Red-cheeked Cordon-bleu, Agahozo-Shalom Youth Village, Rwanda

A bird's blue is not true blue in their feathers. That is, if you ruffle a feather from an Eastern Bluebird, the blue color changes and can disappear. Why?

Eastern Bluebird, near Cleveland, Missouri

According to the Cornell Lab of Ornithology, the blue in nearly all blue birds is a "structural color." The appearance of blue is produced by "tiny air pockets in the barbs of feathers [that] can scatter incoming light." The interlocking barbs and barbules of the feather and air cavities they create form a matrix that “scatters light waves in an orderly fashion, sending only the color blue to the observer,” according to Bird Feathers, by S. David Scott and Casey McFarland. Hence, the structure of the feather displays the color blue.

Great Blue Turaco flight feather, Mabamba Swamp, Uganda
If you look closely at the bottom of the feather, near the fingers,
the disrupted area is brown, not blue!

Indigo Bunting, Blue River corridor, Kansas City, Missouri

This is in contrast to other bird colors like black, gray, brown, red, yellow, pink, and green, which are all caused by pigments in the feathers. Pigments absorb light waves but reflect those that the observer eventually sees. The pigment in a male Northern Cardinal’s feather absorbs other light waves and reflects red. The feather of the Eastern Bluebird does not reflect blue (it reflects grayish-brown), but the structure of the feather scatters light and creates the blue color. To paraphrase the aforementioned authors Scott and McFarland, if you grind up a red feather, it stays red, but if you grind up a blue feather, you lose the blue.