Are fruit colors adapted to consumer vision and birds equally efficient in detecting colorful signals?

Reproduction in plants often requires animal vectors. Fruit and flower colors are traditionally viewed as an adaptation to facilitate detection for pollinators and seed dispersers. This long-standing hypothesis predicts that fruits are easier to detect against their own leaves compared with those of different species. We tested this hypothesis by analyzing the chromatic contrasts between 130 bird-dispersed fruits and their respective backgrounds according to avian vision. From a bird's view, fruits are not more contrasting to their own background than to those of other plant species. Fruit colors are therefore not adapted toward maximized conspicuousness for avian seed dispersers. However, secondary structures associated with fruit displays increase their contrasts. We used fruit colors to assess whether the ultraviolet and violet types of avian visual systems are equally efficient in detecting color signals. In bright light, the chromatic contrasts between fruit and background are stronger for ultraviolet vision. This advantage is due to the lesser overlap in spectral sensitivities of the blue and ultraviolet cones, which disappears in dim light conditions. We suggest that passerines with ultraviolet cones might primarily use epigamic signals that are less conspicuous to their avian predators (presumably with violet vision). Possible examples for such signals are carotenoid-based signals.