| Sumario: | Brood parasites may be favoured over host nestlings due to variation in the honesty of their begging signals. Begging behaviour of great spotted cuckoo nestlings and their host magpie nestlings was recorded when controiiing food need, Cuckoo begging effort was dishonest as an indicator of nutritional need, whilst magpie begging was not. Cuckoos begged for longer and emitted more calls at a higher rate irrespective of the degree of food deprivation, although in contrast to magpies, cuckoos ate food in relation to their need. Energetic and predation costs are unlikely to account for these differences. Differences in indirect inclusive fitness costs can explain the more intensive begging by cuckoos. Magpie parents given a choice favoured larger nestlings and those begging more intensively. Cuckoos obtained more food and a larger share than magpies of a similar size. Magpies therefore received less food in the presence of a cuckoo, and cuckoos received a similar share irrespective of their size. Lack of relatedness to their magpie hosts therefore allows cuckoos to exploit a set of adaptive rules in the host parents and manipulate them into providing the latter with preferential care.
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