Cuckoos are commonly known for brood parasitism, meaning that they lay their eggs in other birds' nests and leaving the unfortunate mother of that nest to care for the invader baby. Sometimes, the cuckoo would even remove the mother's eggs to ensure that the invader baby bird would get the most care and food.
So a cuckoo bird's relationship with a warbler would be that of parasitism. At the very least, it's harmful for the warbler.