Mistletoe is spread by birds like robins, bluebirds, thrushes and cedar waxwings who feed on its berries. The small white to pinkish fruits contain a single seed surrounded by a viscid pulp. Birds eat the fruits, digest the pulp, and poop the still-living seeds about, often on branches of susceptible trees. The sticky droppings may give the germinating seeds a chance to penetrate the host tissues. Others think the sticky seeds attach to the beaks of birds and then adhere to the bark of the host when the birds wipe off the seeds.
Opinion seems to be divided.
Some pathologists think that the severe buildup of mistletoe in some trees but not others is because once an infection starts, the birds, attracted to one fruit-laden parasite, hang about eating and pooping or wiping their beaks in the same tree, so more mistletoes become established, ad infinitum. Heavily infected trees can be weakened and sometimes killed. During drought, weakened trees are predisposed to insect attack and can die. And infected branches are more likely to be broken off in winds. Our Phorodendron is found from the northern Willamette Valley south to Baja California.