Nanostructures in this 40 million year-old feather once made it iridescent. Credit: Jakob Vinther/Yale University

By using an electron microscope to examine nanoscale structures in a 40 million-year-old bird feather, researchers have determined that, in life, the birds were black with an iridescent, bluish-green coppery sheen, like starlings and grackles. The key to figuring this out was the discovery by researchers at Yale University that rod-shaped nanostructures in the feather specimens aren't bacteria, but remnants of pigment-containing cells called melanosomes.

Iridescence in bird feathers is caused by constructive interference of light scattered by the cells; how the light scatters is determined by the arrangement of the melanosomes, which are preserved not only in the bird fossils but in some dinosaur and mammalian ones as well. The Yale researchers hope this technique could be used to get a better picture of the coloring and patterning of dinosaurs and other extinct creatures. This work is described online in the journal Biology Letters.