The relationship between the two is Parasitism (when one organism benefits and the other is harmed) because the Cuckoo bird lays it's eggs in the warblers nest, and the warbler usually cannot tell the difference.
Cuckoo and Warbler. Parasitism occurs when one species benefits from the interaction, and one species is harmed. The warblers often cannot tell the difference between the eggs, so both species are raised together.