The role of chromatic and achromatic signals for fruit detection by birds

Fruit color changes during ripening are typically viewed as an adaptation to increase signal efficacy to seed dispersers. Plants can increase signal efficacy by enhancing chromatic (wavelength related) and/or achromatic (intensity related) contrasts between fruit and background. To assess how these contrasts determine the detectability of fruit signals, we conducted 2 experiments with free-flying crows (Corvus ossifragus) under seminatural conditions in a 2025 m2 aviary. Crows searched first for artificial red and black fruits and detected red fruits from a larger distance. Because artificial red fruits had higher chromatic and lower achromatic contrasts against foliage than artificial black fruits, crows apparently prioritized chromatic contrasts. Thus, the common change in fruit color from red to black during ripening does not increase signal efficacy to crows. In a second trial, crows searched for UV-reflecting and black blueberries (Vaccinum myrtillus) against backgrounds of foliage and sand. Against foliage, UV-reflecting berries had higher chromatic and achromatic contrasts than black berries, and crows detected them from a larger distance. Against sand, UV-reflecting berries had low achromatic contrasts and black berries low chromatic contrasts. Crows detected both fruit types equally, suggesting that they used chromatic contrasts to detect UV-reflecting berries and achromatic contrasts to detect black berries. Birds prioritized chromatic contrasts when searching for artificial red fruits in foliage but not when searching for blueberries on sand. We suggest that the relative importance of chromatic and achromatic contrasts is contingent on the chromatic and achromatic variance of the background. Models of signal perception can be improved by incorporating background-specific effects.