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 food, but the complexity of the display is poorly understood. Here we explain the function of reed warbler begging signals and show how they are exploited by the common cuckoo, Cuculus canorus , a brood parasite. Reed warbler parents integrate visual and vocal signals from their young to adjust their provisioning rates, and the two signals convey more accurate information about offspring need than either does alone. The cuckoo chick has a particularly 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 display. Therefore the cuckoo succeeds not through mimicry of the host brood begging signals, but by tuning into the sensory predispositions of its hosts.