For these comical-looking birds, an exquisite handlebar mustache is more than a fashion statement—it’s an advertisement of good health. Like all other birds, these terns can only grow out their plumes while molting, an extremely energy-intensive process during which they sequentially replace all of the feathers on their bodies. This allows them to use the unique facial feathers to assess the fitness of prospective mates: since growing a pair of long ornamental feathers requires a surplus of food, birds with longer mustaches are better at feeding themselves and are therefore likely to be better at raising young.
Excerpt from "Birds" by Tim Flach, texts by Richard O. Prum, Abrams, 2020