Precise targeting host activity for parasitism by common cuckoos

This study was carried out from March to August 2018–2025 in the Liuzhi area (26°13′ N, 105°42′ E), Guizhou, southwestern China. The study area mainly consists of a karst landscape, with a mosaic of villages, farmland, shrubland, and barren slopes. The preservation of traditional agricultural practices may play an important role in the distribution and reproduction of birds in the area (Zhong et al., 2023).The redstart is a small passerine songbird belonging to the family Muscicapidae (Zheng 2023). In the study area, its breeding season lasts from late March to mid-August. This species typically raises 1–2 broods per year, with a clutch size of five eggs (ranging from 3 to 6). Eggs are pink or pale blue with brown spots; however, there is substantial phenotypic variation among clutches (Wan et al., 2025). Nest sites are highly variable, spanning a continuum from open nests to secondary cavities. Moreover, most breeding individuals build nests on artificial objects—such as buildings, household items and even cars—reflecting a unique adaptation to cohabitation with humans (Figure 1, Video S2-S8). Common cuckoos parasitizing redstarts lay pure blue eggs and pale blue eggs with brown spots. The former exhibits egg phenotype mimicry with the pink eggs of the redstart (Wan et al., 2025). Additionally, other major secondary cavity-nesting species in the area include the Oriental magpie-robin (Copsychus saularis)—which lays blue, brown-spotted eggs, distinct from the cuckoo's pink eggs (Figure 3a)—and the tree sparrow (Passer montanus).Locating and successfully parasitizing host nests is crucial for the adaptive evolution of parasitic birds. However, how cuckoos (Cuculus spp.) locate hosts with highly variable and concealed nest sites 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 that although Daurian redstarts nest in a variety of artificial structures, common cuckoos are able to track host activity and parasitize nests located inside houses, cars, beehives, and other human-made installations. Moreover, more exposed nestboxes experienced higher parasitism rates, and cuckoos only parasitized boxes with active host attendance. 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.