Anti-parasitic egg rejection by great reed warblers (Acrocephalus arundinaceus) tracks differences along an eggshesll color gradient.

•Avian brood parasite hosts reject foreign eggs based on color contrasts between own and foreign eggs.•Traditional models assume that hosts use absolute color contrasts to reject foreign eggs.•Great reed warblers use directional color contrasts, more likely to reject browner than bluer eggs.•Directional color contrasts increasingly shown to explain egg rejection in diverse host species.