The Phainopepla, a midsized migrating songbird and member of the silky flycatcher family, inhabits dry regions of the southwestern U.S. and central Mexico. Its range reflects a close relationship that has evolved over time with mistletoe, a partially parasitic plant that grows on mesquite trees and desert shrubs. The Phainopepla feeds on insects like most other birds, but its winter diet consists chiefly of mistletoe berries. The Phainopepla's migration pattern is tied to the life cycle of mistletoe. The birds time their arrival in the desert just as mistletoe berries are ripening, typically in October. The Phainopepla remains in the desert until May, when it leaves for parts that remain largely unknown to scientists.

The Phainopepla and the mistletoe share a symbiotic relationship, which is a close ecological relationship between the individuals of two or more different species. These individuals live in close proximity to one another. In fact, sometimes one individual lives inside the body of the other. In some symbiotic relationships, both species benefit. This is called mutualistic symbiosis, or mutualism. In the case of the Phainopepla and mistletoe, the plant supplies food to the bird, and the bird, in turn, helps the plant reproduce within their shared habitat. This happens in one of two ways. As the Phainopepla eats mistletoe berries, it may wipe seeds from its bill on a live tree or shrub branch. Alternatively, seeds that are consumed along with the fruit pass through the bird's digestive tract and are dropped onto limbs below the bird's perch.

Mistletoe bears leaves for photosynthesis, but it requires another plant to supply it with water and mineral nutrients. Plants like mistletoe that obtain some nourishment from their host but also photosynthesize are called hemiparasitic. Because the mistletoe's seeds are coated with viscin, a sticky material that hardens as it dries, the seeds adhere to branches of potential new hosts. In other parts of the world, mistletoe shares a mutualistic symbiotic relationship with other bird species. In Europe, the species is the Mistle Thrush, and in Asia and Australia, it's the Dicaeum.

Examples of symbiosis are found throughout the natural world, but not all forms of it are mutually beneficial. Parasitism, which describes the relationship between mistletoe and its host tree or shrub, is a relationship in which one individual provides food or shelter for another individual, the parasite. The parasite enjoys full the benefit of this relationship, while the host is harmed. If mistletoe becomes overgrown on a host plant, it will kill it—and, as a result, will also kill itself. Many worms, such as tapeworms, are also parasites. They feed off—and shelter inside—other animals, including humans. Commensalism describes a symbiotic relationship that benefits only one of the individuals and neither helps nor harms the other. For example, barnacles attach to whales, which carry the barnacles to places where food is abundant.

To learn more about symbiotic relationships in nature, check out Ancient Farmers of the Amazon and E. O. Wilson: Ants and Ecosystems.

To learn about other animals that inhabit the desert southwest, check out Bighorn Sheep, Bats of the Southwest, Burrowing Owl, Horned Lizard, and Tarantulas.

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