Random egg-laying in host nests by cuckoos

This study was carried out at Sifangtuozi Farm (46°12ʹ N, 123°84ʹ E), Baicheng City, Jilin Province, northeastern China, during June to August 2023, which is the breeding season of the ORW and cuckoos. The area is a typical reed wetland, and the ORW nests on reeds. They are the dominant species in the area and serve as the sole host of the common cuckoo there (Wang et al., 2023). At the study site, cuckoo parasitism rates reach as high as 47%–58% (Wang et al., 2022).Obligate brood parasitic cuckoos (Cuculus spp.) engage in a reproductive strategy of laying eggs in host nests, thereby effectively delegating the responsibilities of incubation and chick-rearing to other host species. The common cuckoo (Cuculus canorus) is a brood parasitism “generalist,” has historically been a central subject of research on coevolution with its various hosts. An ongoing debate exists regarding whether common cuckoos choose host nests following an egg matching strategy. By using three groups of field experiments, this study examined how the common cuckoo lays parasitic eggs in nests of its common host, the Oriental reed warbler (ORW) (Acrocephalus orientalis). Our results demonstrated that the common cuckoo did not exhibit any notable selection preference for experimental nests containing different types of eggs. Thus, our findings provide direct experimental evidence in support of the random egg-laying hypothesis by cuckoos.