Cuckoo eyes are an important identification cue for the Oriental reed warbler host

Successful recognition of parasites through effective identification cues can reduce the cost of anti-parasitic defenses by the host. Features on the front of the body such as the eyes may be important clues for the prey to perceive and recognize the parasite. In this study, we investigated whether the eyes of common cuckoos (<i>Cuculus canorus</i>), an obligate avian brood parasite, served as identification cues for its common host, the Oriental reed warbler (<i>Acrocephalus orientalis</i>). We displayed dummies of 3D printed common cuckoos and Oriental turtle doves (<i>Streptopelia orientalis</i>) with their eyes covered or not-covered near the nests of breeding Oriental reed warblers to test effect of the eyes on defense behaviors of the warblers towards these dummies. Oriental reed warblers significantly reduced attacks on the common cuckoos with eyes covered compared to those with eyes not-covered. However, there was no significant difference between the attacks on the Oriental turtle doves with not-covered eyes and those with covered eyes. Our results suggest that Oriental reed warblers use cuckoo eyes as an important discrimination cue. We explored for the first time the use of the cuckoo's eye as an important discriminative cue by Oriental reed warblers via visual manipulation of 3D printed cuckoos, which provides a new experimental validation of the host Oriental reed warbler's anti-parasite strategy in terms of visual cues. However, Future research should consider testing more cuckoo species and their hosts and further validating the identification cues of the eyes and other body parts in additional geographic populations.