Mistletoe grows high up in trees in North America and Europe. The mistletoe plant is considered to be a semi-parasitic to parasitic evergreen.
While it is toxic to most small animals, mistletoe berries are eaten by birds. Birds then roost in tree branches and drop seed-filled excrement on the branches of oak, apple and fir trees. The mistletoe seeds sprout, take root in the tree branches and feed on the tree branches, which is why mistletoe is classified as parasitic.
From the ground, it looks as if a ball of leaves resembling squirrel's nests are alive in the tree's upper branches. It is easiest to spot mistletoe in winter, when hardwood trees have gone dormant and tree leaves are gone, because mistletoe retains it greenish yellow color.