DOES THE GREAT SPOTTED CUCKOO CHOOSE MAGPIE HOSTS ACCORDING TO THEIR PARENTING ABILITY

When brood parasites are about to lay an egg, they have to decide whic h nest to parasitize. The best nest in which to lay will depend on the parenting ability of the host. We have studied selection of magpie (P ica pica) hosts by great spotted cuckoos (Clamator glandarius). Great spotted cuckoos preferentially parasitize large host nests. Nest volum e in magpies is a good indicator of territory quality, since there is a negative relationship between magpie nest size and breeding date, an d timing of breeding in magpies is known to be positively related to t erritory quality. Moreover, magpies occupying high-quality territories have high breeding success. Therefore, nest size is positively relate d to the quality of magpies. Parasitized magpie nests were of greater volume than the nearest neighbouring nest not parasitized by the great spotted cuckoo. In order to test whether the great spotted cuckoos mi ght select high-quality magpie hosts, we manipulated pairs of parasiti zed and non-parasitized nests with identical laying dates and habitats , introducing into each of the nests the same number of parasitic and non-parasitic eggs, The number of fledglings reared (magpie plus great spotted cuckoo chicks) in naturally parasitized nests was higher than in experimentally parasitized nests. Thus, the probability of surviva l of the parasite chicks increased if cuckoo eggs were laid in the nes ts of high-quality hosts originally chosen by the parasite.