<b>Precise targeting host </b><b>activity </b><b>for parasitism by common cuckoos</b>

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). Previous monitoring in this population indicates that 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) (Wan et al., 2025). 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 (Figure 2). The latter closely resembles the pink eggs of the redstart in phenotype (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 Eurasian tree sparrow (Passer montanus). Field data collection We monitored the breeding activities of redstarts within the study area (∼1800 ha) by locating nests and recording parasitism events by the common cuckoo. When finding the host's nest, we recorded the breeding status and types of the breeding site. We then conducted regular checks and video‑recorded cuckoo visits to these nests. Given that human-adapted Daurian redstarts may gain anti-parasitic benefits from nesting in anthropogenic shelters, we classified natural nest sites along two dimensions: (1) based on the possibility that cuckoos avoid approaching buildings (Liang et al. 2013; Zhang et al. 2023), nests were recorded as either "on houses" or "non-houses"; (2) given that cuckoos do parasitize indoor nests and that some hosts prefer breeding indoors (Figure 1), we further categorized nest sites as "indoors" or "outdoors." This classification was designed to investigate what cues cuckoos use to locate indoor target nests. Furthermore, cases in which a cuckoo removed a host egg but did not lay its own were also classified as parasitism events rather than simple predation, as they may reflect a significant parasitic strategy known as “farming behavior” (Zhong et al., 2019; Zhang et al., 2024). To test the relationship between cuckoo selection of redstarts and nest concealment as well as host activity cues, we conducted a nestbox attraction experiment from 2019 to 2023 and in 2025 by attracting some redstarts and other species to breed in nestboxes. Compared to natural nest sites, these nestboxes, with their unobstructed host activity, were more easily detected and parasitized by cuckoos. The internal dimensions of the nestbox were 12 cm × 12 cm × 26 cm (length × width × height). To ensure that the cuckoo can successfully enter, the nest entrance was 10 cm × 8 cm (length × width). The nestboxes were placed in or near residential areas, on utility poles, trees, or the walls of houses, at a height of at least 3 meters above the ground. Nests were checked every 5–6 days during the early breeding season. When breeding pairs were observed in the nestbox, we checked for egg laying and cuckoo parasitism every 2–3 days depending on the nesting progress. Due to the strong nestbox occupancy behavior of the Eurasian tree sparrow, we removed nest material from some sparrow nests during the early nesting period to increase occupancy by redstarts and oriental magpie-robins (Wan et al., 2023). In addition, sparrows typically fill the interior of nestboxes with nesting material and leave small entrances and exits immediately adjacent to the walls of the nestbox, making it virtually impossible for cuckoos to enter the interior of the nest to lay eggs (and natural sparrow nests are not accessible to cuckoos for egg laying); accordingly, we did not consider sparrows as a potential host for cuckoos in this study. 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.