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.

Brood parasites, such as the Common Cuckoo (Cuculus canorus), rely on manipulating hosts from other species to raise their offspring. Selection should favor individuals that are able to identify and choose host nests that closely match the incubation stage of their own eggs. While intuitive, and supported by the findings of numerous long-term monitoring studies, experimental evidence of this remains sparse. By using video monitoring, we used three experimental groups to explore whether Common Cuckoos preferentially choose host Oriental Reed Warbler (Acrocephalus orientalis) nests that are early in their laying cycle. We found that Common Cuckoos preferentially parasitize nests of the host Oriental Reed Warbler that are earlier in their laying (less vs. more eggs) or nesting cycle (eggs vs. chicks), and videos of our experiments provide direct insights into their decision making process. Our results provide strong experimental evidence that Common Cuckoos are able to assess nesting stage and strategically choose nests that increase the likelihood of successful parasitism.