An example of a parasitism relationship is that of ticks, fleas, lice or leeches on a host such as a human or dog. A parasitism relationship is where one of two plants or animals gains at the expense the other without killing it.
Another form of parasitism can be seen with brood parasites where eggs are laid in another bird or insect's nest so that the other species will hatch and raise the bird. The common cuckoo parasitizes a wide range of different birds including reed warblers and dunnocks. The color of each egg depends on the mother's genes, so that each different cuckoo, and all of its future offspring, specializes in parasitizing a certain species of bird.