Measuring up to 9 inches (23 cm) long, the bill of a toco toucan accounts for a third of its total body length, making it the largest bill relative to body size of any bird. Although the birds use them in a variety of ways, they are believed to have initially evolved as a sexual display: toucans and their closest relatives all have extremely colorful bills, and they use them extensively during their courtship rituals, in which the male toucan presents the female with a gift—usually a large nut or piece of fruit—and then carefully uses his beak to place it inside her bill.
Excerpt from "Birds" by Tim Flach, texts by Richard O. Prum, Abrams, 2021