The number of weeds thus eliminated from one farm in a year is enormous. Not all seeds eaten by birds are destroyed. Many pass through the digestive tract uninjured. They sprout and take root. Birds may also eat fruit but spit out the seeds inside the fruit.
Birds also carry seeds in mud clinging to their feet.
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.