Living on the edge: background borders enhance camouflage despite imperfect match

Background matching, a camouflage strategy where animals match the colours and patterns of their background, is background-specific. However, many habitats have multiple backgrounds, with sharp borders delineating the distinct backgrounds. When an animal rests on the border between a matching and a non-matching background, its overall background match is reduced. However, the animal may be better concealed because of increased visual clutter and the introduction of additional edges. We tested the effectiveness of background borders in experiments where birds and human "predators" were tasked with finding targets on artificial backgrounds. Both birds and humans took longer to detect targets placed on background borders than those placed on matching backgrounds. Thus, prey positioned at the intersection of two background types may be able to avoid detection despite having a low background match. Strikingly, birds took longer to find targets placed on a non-matching background than on a matching background, probably because they mistook targets for a patch of the matching background. Our findings indicate that prey can conceal themselves despite poorly matching their local background if they can match backgrounds in the immediate vicinity that are also part of the predator's visual field.