How selfish is a cuckoo chick?

We studied the begging display of nestling cuckoos, Cuculus canorus, reared by reed warbler,Acrocephalus scirpaceus , hosts, to test various hypotheses for the cuckoo's unusually rapid begging call. The hypotheses are not mutually exclusive but focus on different parts of the chain: chick need–begging signals–provisioning by hosts. We reject two hypotheses. (1) Cuckoo chicks do not use their exaggerated begging to counteract host rejection: begging displays varied with hunger and functioned entirely to solicit food. (2) Cuckoos also do not exaggerate their begging calls simply because they need more food than a host brood. Single cuckoos grew at a similar rate to a brood of four reed warblers, and more slowly than a blackbird, Turdus merula, chick (a nonparasitic chick of similar size). Our data support two other hypotheses. (3) To elicit sufficient care in reed warbler nests, the cuckoo must exaggerate the vocal component of its display to compensate for its deficient visual signal (a single gape) compared with a host brood. Thus rapid calling reflects the way the cuckoo exploits the provisioning rules that hosts use to feed their own young. (4) In theory, cuckoos should be more selfish than host young because their greed is unconstrained by kinship. Our data are equivocal; compared with host broods, cuckoos solicited a higher provisioning rate in relation to one measure of need but not for another. We discuss whether cuckoos are likely to have gens-specific begging displays.