Instinct

Some animals have evolved the capacity to take advantage of the reliable, instinctive behavior of others. Avian brood parasites, including the North American cowbird and the European cuckoo, exploit the parental behavior of other birds and lay their eggs in the host's nest. The unwitting host feeds the interloper's hatchlings, which are often bigger than its own, and thus may represent a greater releaser of the powerfully instinctive feeding behavior of the parents. The adult brood parasite is literally parasitizing the parental behavior of the host bird, for it exerts no further parental investment in its offspring, leaving them instead in the care of the host.