Egg colour influences egg removal by common cuckoos (Cuculus canorus)

The data collection was performed in Sifangtuozi farm (46°12ʹ N, 123°84ʹ E), western Jilin, Northeast China in 2022. Field experiments were conducted from June to August, which is the peak of the ORW breeding season. ORW build their nests preferably in reeds and are the main host species of the common cuckoo in this area (Wang et al., 2022a; Zhao et al. 2022). We systematically searched for ORW nests in reed habitat where we observed host activity. When we found the nest, we recorded its building stage (completed or within the process of building), clutch size and cuckoo parasitism (presence or absence). Once the ORW female laid the first egg in the nest, we assigned the nest to one of the two experimental groups. For group 1, an experimental nest containing one red and one green model egg was tied with thin wire one meter away from the active ORW nest (Figure 1; group 1). For group 2, two experimental nests, one containing two red eggs and the other containing two green eggs, were glued with thin wire together and tied one meter away from the ORW nest (Figure 1; group 2). Since we previously found that larger nests have higher chances of being parasitized (Wang et al. 2022a), both experimental nests of group 2 were of the same size and shape. The orientation of the two nests was randomized as we are not aware whether cuckoos approach nest from a particular direction. Both groups of experiments were designed to monitor the colour preference of cuckoos when removing a model egg during parasitism. In addition, the second experiment investigated the strength of the motivation for a particular egg colour because the two model egg types in this group were in separate nests, further from each other than in group 1 (see Figure 1). Experiments from both groups were carried out near different host nests, meaning every host nest was used only once. The experimental nests used in the study were old ORW nests collected at the end of the previous breeding season and reinforced with beige sewing threads. Experimental nests were placed approximately 1.0 m above the active host nests to encourage cuckoo parasitism, as cuckoos tend to parasitize nests that are located higher up (Clarke et al. 2001; Patten et al. 2011).Recent studies that investigated cognition mechanisms underlying egg removal found that cuckoos preferentially remove brighter eggs because darker eggs may be more cryptic. However, the role of chromatic differences (e.g., in hue) has been overlooked, despite being key cues in avian egg recognition. Using red and green model eggs and video recordings of experimental nests, we found that common cuckoos (Cuculus canorus), preferred removing red model eggs in 92% of cases. Such a strong preference for eggs of a certain hue indicates that cuckoos use chromatic cues to selectively remove eggs from host nests. Avian visual modelling revealed that it is unlikely that the preference for red eggs was elicited by higher conspicuousness of red eggs in nests. We speculate that cuckoos might have been attracted to red-coloured eggs similarly to how other bird species are attracted to red-coloured diet. These findings contribute to better understand the purpose of egg removal in brood parasites.