Hummingbirds became the fluttering jewels of the animal kingdom by evolving innovative ways of producing shimmering colour.
Many birds owe their colours in part to tiny organelles called melanosomes, which are embedded in bird feathers and produce pigments. Melansomes also reflect light in a way that creates iridescence in ducks, hummingbirds and other birds.
Chad Eliason at the Field Museum in Chicago, Illinois, and his colleagues compared melanosomes and other feather structures among 34 hummingbird species. The scientists found that hummingbird melanosomes are stuffed with tiny air bubbles, and are stacked on top of one another. Variation in the sizes of both the bubbles and the stacks alters the feathers’ hue and colour saturation, and transforms the feathers’ reflective surfaces and thus their iridescence. In addition, the thickness of the keratin that coats the feathers helps to determine the wavelength of light that the feathers reflect.
The authors conclude that hummingbirds have a rainbow of colours because of the many tweaks to each of these feather characteristics over evolutionary time.