U.S. birds display a wide array of striking hues. But just how colorful are our American birds? What you need to know from our study.

Eocoracias brachyptera, a bird species that lived some 48 million years ago, was blue—the first time that blue feathers have been reconstructed from the fossil record.

The birds are plumed virtually in black, not blue. And because the blue is a light show rather than pigmentation, the birds can appear to have several shades of the color. Young males look like ...

Instead, the blue sported by bluebirds comes from tiny air pockets inside the feathers, which scatter light to create blue. Red feathers, however, rely on pigment alone.

While humans have three color cones in the retina sensitive to red, green and blue light, birds have a fourth color cone that can detect ultraviolet light. A Princeton-led research team trained wild ...