Feathers define birds as we know them. Whether fanned in courtship display, slicing the air, or gliding through water, birds’ plumage makes them unique among living animals. But as fascinating and glorious as feathers may be, they are merely dead tissue, and any damage to them is permanent. Too many impaired feathers spells disaster for a bird. Luckily, birds can replace their mangled plumes—and they do so at least once a year in a process called molting, often right under the noses of birders. Some species take the opportunity to sport seasonal looks, like the nearly radioactive red of a male Scarlet Tanager in spring. But most importantly, molt is a survival tactic, necessary to keep feathers in peak condition. Simply put: If a bird doesn’t molt, it will die. When, where, and how often a bird replaces its plumes varies widely between and even within species, making molt a formidable research subject, though one that is key to understanding a species’ life history and...