PREFERENTIAL ALLOCATION OF FOOD BY MAGPIES PICA-PICA TO GREAT SPOTTEDCUCKOO CLAMATOR GLANDARIUS CHICKS

Adult magpies Pica pica provide parasitic great spotted cuckoo Clamato r glandarius nestlings with a diet very similar to that fed to their o wn chicks. In both naturally and experimentally parasitized nests, gre at spotted cuckoo chicks were fed at a higher rate than magpie chicks in the same nest. This preferential allocation of food by magpie paren ts to great spotted cuckoo chicks is consistent with the supernormal s timulus hypothesis, because this result implies that cuckoo chicks pro vide stronger stimuli for parental care than host chicks. Great spotte d cuckoo chicks receive most of the food brought to the nest by the fo ster parents, because they exploit a series of stimuli which jointly ( or sometimes individually) operate as a supernormal stimulus. This hyp othesis predicts that if ally stimulus is masked, the efficiency of th e cuckoo in eliciting parental care will decrease. Here, we analyze ex perimentally the effects of two of these stimuli, preferential feeding of large nestlings and of nestlings with conspicuous palatal papillae . Firstly, when we experimentally introduced one medium-sized (7-9 day s) cuckoo chick into an ur;parasitized magpie nest where the largest m agpie chick was 12-15 days old, the cuckoo did not receive significant ly more food than the average or the largest magpie chick. Secondly, w hen unparasitized nests were experimentally parasitized with a cuckoo chick that had its gape painted to mimic that of magpie chicks, the pa rasitic cuckoo received less food than the average magpie chick.