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A chick of the Horsfield's hawk cuckoo Cuculus fugax has a vivid yellow skin patch on each wing-bend, which is very similar in colour to its own gape. Almost every time a host parent delivers food, the chick explicitly displays the wing-patch by raising and shaking the wing. We observed that host parents tried to place food onto the wing-patch of the nestling instead of into its mouth. The wing-patch would be perceived as a gape by host parents, possibly resulting in an overestimation of brood size. Signals induced by the wing-patch may vary with the luminosity of environments.