The naturalist Charles Darwin reared 82 plants from one ball of earth gathered from the foot of a partridge. Mistletoe is completely dependent on birds such as cedar waxwings for its establishment in new areas. Birds eat the berries and deposit the seeds later, after they have passed through the bird's digestive tract, in trees that serve as hosts. Many abandoned old fields spring to new life with trees, flowers, and weeds that have grown from seeds scattered by birds. The destruction of rodents is another service birds perform for the farmer.
Rodents feed chiefly on roots, seeds, and green crops.
The damage they do amounts to millions of dollars yearly. Hawks, owls, and other predatory birds are one of nature's checks upon the number of rodents. Each hawk or owl requires about three mice a day. Owls are far more efficient than cats at clearing out a rodent-infested barn. As humans have cut down trees and cleared land for building cities, they have destroyed many of the places that birds normally use for nesting.