Oriental reed warblers recognize true and model eggs of various colors

The present study was conducted at the Sifangtuozi Farm (46° 00′–46° 22′ N, 123° 46′–123° 57′ E) in Jilin Province, China. During the breeding season (June–August) each year, large numbers of ORWs migrate to reed beds surrounding farmland ridges, aqueducts, ditches, and ponds in the region to breed. ORWs are among the main hosts of CCs, with parasitism rates of 34.3%–65.5% (Yang et al. 2016, 2017; Wang et al. 2021). Field experiments were conducted in the study area from June to August in 2022 and 2023.Experimental designNests were visited at least once a day after the initiation of all egg experiments. Images and videos of each nest were obtained, and a micro video recorder (Ou Chuang A8; Xiamen Shangyu Huajin Electronic Technology Co., Ltd., Xiamen, China) was used for real-time recording of the experimental status when necessary. Behavioral responses were monitored for 6 consecutive days. Foreign eggs were considered accepted if they remained intact in the nest on day 6 and the ORWs continued breeding in the nest and rejected if the foreign eggs exhibited peck marks, were punctured, buried, or ejected, or if the nest was abandoned (Moksnes et al. 1991; Moskát and Hauber 2007; Wang et al. 2015). All nests artificially installed in the experiment were abandoned ORW nests collected in the same year or ORW nests collected after the breeding season of the previous year. ORW eggs used in the experiments were conspecific eggs collected from abandoned nests, and commercially available budgerigar (Melopsittacus undulatus) eggs were used as foreign eggs (true eggs). All model eggs were artificial eggs made by hand using commercially available polymer clay.In this study, we investigated the egg recognition and rejection responses of the Oriental reed warbler (Acrocephalus orientalis, ORW), a common host of the common cuckoo (Cuculus canorus, CC), to foreign eggs of different colors and types (true vs. model eggs). The aims were to analyze the influence of foreign egg color on the egg rejection of ORWs and to compare the recognition of true and model eggs. ORWs rejected almost all foreign eggs of different colors, with the majority of foreign eggs being removed via ejection. The difference in the rejection of true eggs vs. model eggs by ORWs was not significant. These findings indicated that ORWs possess the ability to accurately recognize conspecific eggs and reject all eggs differing in color, with no significant differences in rates of recognition and rejection between true and model eggs. Future research should involve a larger sample size and conduct comparisons of populations spanning a broader geographical area.