Three kinds of U.S. butterflies depend on mistletoe for survival: the great purple hairstreak, the thicket hairstreak and the Johnson's hairstreak. These butterflies lay eggs on mistletoe, and their young eat the leaves. The adults of all three species feed on mistletoe nectar, as do some species of native bees.
The mistletoe's white berries are toxic to humans but are favored during autumn and winter — when other foods are scarce — by mammals ranging from deer and elk to squirrels, chipmunks and porcupines. Many bird species, such as robins, chickadees, bluebirds and mourning doves, also eat the berries.
The kissing custom may date to at least the 1500s in Europe. It was practiced in the early United States: Washington Irving referred to it in “Christmas Eve,” from his 1820 collection of essays and stories, The Sketch Book of Geoffrey Crayon, Gent. In Irving's day, each time a couple kissed under a mistletoe sprig, they removed one of the white berries. When the berries were gone, so was the sprig's kissin' power.