Green LIght, Red Light

Day 17: Remember that game you played as a kid called "Red Light, Green Light?" Hummingbirds like this Anna's have it built in. This remarkable ability to change colour seemingly on a whim is due to the physical structure of the feathers rather than to pigmentation. Microscopic layers within the feather only permit passage of specific wavelengths, and as light strikes the feather from different angles, multiple microlayers combine or cancel specific wavelengths to produce the visual effect of colour change. A hummingbird's shimmering throat feathers may hold as many as fifteen different microlayers, each with a thickness which matches one colour of light. The reflected light we perceive as iridescence is the product of amplification of some colours, reduction of others. With the ability to go from red to green with just a shift of the head, it's no wonder hummingbirds can halt so abruptly mid-air and then be gone in a flash. They've had lots of practice playing "Stop and Go."