High synchrony of egg laying in common cuckoos (Cuculus canorus) and their great reed warbler (Acrocephalus arundinaceus) hosts

Female common cuckoos Cuculus canorus lay eggs every other day and typically parasitise nests before the hosts complete their clutches. When laying strategy of the cuckoo is well-synchronised with that of the host, the reproductive success of the parasite may increase. In contrast, when parasitism frequencies are high, cuckoo females might be less able to find suitable nests of the desired stage and without any prior cuckoo eggs. Contrary to this prediction, we found high synchrony of laying patterns between cuckoos and their local primary host, the great reed warbler Acrocephalus arundinaceus, under unusually heavy brood parasitism (ca 64% parasitism frequency) on the Hungarian Great Plane. Although the laying activities of the hosts showed different temporal patterns in the 2 consecutive years studied, the cuckoos closely followed the patterns in each year. This occurred despite the arrival of the cuckoos at the study site 2-3 weeks earlier than the great reed warblers. The matching of laying patterns with those of the hosts suggests an adaptive response to both ensure optimal hatchability of the cuckoo eggs and to avoid multiple parasitism of the same nest even under heavy pressures of brood parasitism.