Precisely targeting host activity for parasitism by Common Cuckoos

The evolution of parasitic strategies through tracking host life-history patterns represents a key adaptive trait in avian brood parasites. However, when hosts successfully exploit human-modified environments—such as diverse and concealed nesting sites—the mechanisms enabling cuckoos ( Cuculus spp.) to accurately locate and parasitize such nests remains unclear. The Daurian Redstart ( Phoenicurus auroreus ), a human-commensal secondary cavity-nester, often breeds in diverse artificial structures—leading to variable and concealed nest locations—making it an ideal system to study this question. Using long-term field monitoring and a nestbox attraction experiment, we investigated how Common Cuckoos ( Cuculus canorus ) locate and parasitize such concealed nests. Our results show no significant variation in Common Cuckoo parasitism rates across different natural nesting sites. However, nestboxes that were more exposed experienced higher parasitism rates than natural nests. Furthermore, cuckoos targeted exclusively those boxes with active host attendance, rather than boxes occupied by other species. This study provides the first evidence of cuckoo parasitism on a human-associated host nesting indoors, underscoring the key role of host activity cues and suggesting that cuckoos, like their hosts, are also adapting to anthropogenic refuges.