Signals of need in parent-offspring communication and their exploitation by the common cuckoo

Nestling birds present vivid gapes and produce loud calls as they solicit f ood, but the complexity of the display is poorly understood, Here we explai n the function of reed warbler begging signals and show how they are exploi ted by the common cuckoo, Cuculus canorus, a brood parasite. Reed warbler p arents integrate visual and vocal signals from their young to adjust their provisioning rates, and the two signals convey more accurate information ab out offspring need than either does alone. The cuckoo chick has a particula rly striking begging display which has been suggested to be irresistible to host parents. However, we show that the cuckoo, reared alone in the nest, presents a deficient visual display, and elicits the same amount of care as a reed warbler brood only by compensating with its exaggerated vocal displ ay. Therefore the cuckoo succeeds not through mimicry of the host brood beg ging signals, but by tuning into the sensory predispositions of its hosts.