Christmas season in the Northern Hemisphere coincides with the winter solstice, bringing the darkest of cold, gray days. Many Americans compensate for the gloom outside by setting up a cheerful holiday scene inside: the fire dances, candles flicker, gold glints, silver glimmers, holly glistens, poinsettias pose. Lording over the merry panoply is the Christmas tree, spangled with lights, ornaments, tinsel and crowned by a shimmering star.

But lately, when visiting friends for December merrymaking, I’ve noticed something missing: mistletoe. This poetic holiday adornment—a sprig of green leaves and white berries happily dangling over the door—is increasingly conspicuous by its absence, leaving many a rosy cheek unkissed. Where is the love?

Heaven knows, humankind is a fickle lot. Yet mistletoe has been valued for millennia. The plant was prized by ancient Greeks, Romans, Celts and Norsemen for its purportedly magical properties. In those bygone days, people were rightly inspired by mistletoe’s season-defying vitality in the stark, denuded winter landscape. High in the treetops, it merrily flourished, a blaze of bright foliage and gleaming berries.

Druids believed that mistletoe banished evil and promoted animal and human fertility. Ancient Greeks thought it was an aphrodisiac. Romans endowed it with healing powers. Later on, truculent Vikings associated it with peace. When enemies had a chance encounter beneath a mistletoe-laden tree, they would lay down their arms and keep a truce until the following day.

Over time, the custom evolved to suspend mistletoe over a home’s entrance, a talisman of good will. Peace now given a chance, the British upgraded the tradition to kissing under the mistletoe, believing that doing so augured marriage. Even if we moderns are sometimes suspicious of a Christmas kiss, what’s the harm in continuing to hang legendary greenery?

We moderns might also complain that mistletoe, far from being the charming emblem of legend, is a parasite. Yes, the plant thrives by siphoning fluids and vital minerals from host trees, causing them to decline and fall. Nonetheless, mistletoe is a parasite with benefits, more Robin Hood than Robber Baron. Scientists have even designated it as a keystone species, meaning one that is crucial to its ecosystem.

Mistletoe’s berries and flowers are especially attractive to birds, who not only feed on its fruits and seeds but are apt to take up residence in its dense, evergreen clumps, called “witches’ brooms.” These 2-to-3 foot whorls of stems and leaves, which dangle from tree branches, are like an Airbnb for the avian crowd. Owls especially like mistletoe, though insects and discerning small mammals find it cozy as well.

In Australia, 75% of arboreal nesting birds live in witches’ brooms¹. In southwestern Oregon, 90% of the endangered owls are contented broom residents². Rather than banishing mistletoe, conservationists are trying to preserve these crucial habitats. Indeed, mistletoe is listed as an endangered species. Tell a Northern Spotted Owl that you consider mistletoe a parasite, and it may hoot you out of town.

So raise a glass to this oft-misunderstood natural benefactor—and put the mistletoe back atop the door where it belongs.

A version of this article appeared in The Wall Street Journal on December 21st, 2017