Feather colors are generally believed to be relatively static, changing only by small amounts over periods of months. However, a recent study argues that since feathers are neither innervated nor vascularized, color changes must be caused by external stimuli. Researchers analyzed the ways in which feathers of iridescent mourning doves responded to stimulus changes of adding and evaporating water. As a result, it was discovered that iridescent feather color changed hue, became more chromatic, and increased in overall reflectance by almost 50%. Transmission electron microscopy and transfer matrix thin-film models revealed that color is produced by thin-film interference from a single layer of keratin around the edge of feather barbules, under which lies a layer of air and melanosomes. Once the environmental conditions were changed, the most striking morphological difference was a twisting of colored barbules that exposed more of their surface area for reflection, which explains the observed increase in brightness. Overall, the researchers suggest that some plumage colors may be more changeable than previously thought possible.