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When should Common Cuckoos Cuculus canorus lay their eggs in host nests?

Geltsch, Nikoletta, Bán, Miklós, Hauber, Mark E., Moskát, Csaba
Bird study 2016 v.63 no.1 pp. 46-51
Acrocephalus arundinaceus, Cuculus canorus, chicks, eggs, hatching, hosts, nestlings, nests
Capsule Brood parasitic Common Cuckoo Cuculus canorus chicks hatch earlier than the nestlings of their Great Reed Warbler Acrocephalus arundinaceus hosts, but hatching priority is less consistent when Cuckoo eggs are laid after the onset of host incubation. Aim To reveal by field observations what the optimal stage is for Cuckoos to lay their eggs in relation to the host laying cycle to ensure prior hatching of the parasitic chicks. Methods We monitored the hatching of Cuckoo chicks in relation to the hosts’ laying stage at which the Cuckoo eggs appeared and also monitored host incubation behaviour. Results Great Reed Warblers incubated more on day 5 after the host's onset of laying relative to day 3. All Cuckoo eggs hatched earlier than hosts when they were laid prior to the onset of host incubation (day 4). Cuckoo eggs also maintained hatching priority in about 2/3 of the nests when laid on days 5–6. Conclusions Most Cuckoo eggs are laid prior to the onset of host incubation and this, together with other adaptive mechanisms, ensures the prior hatching of Cuckoo eggs. Cuckoo eggs laid after the onset of incubation lose the advantage of prior hatching in approximately 30% of nests.