Shades of red: bird-pollinated flowers target the specific colour discrimination abilities of avian vision

Colour signals are a major cue in putative pollination syndromes. There is evidence that the reflectance spectra of many flowers target the distinctive visual discrimination abilities of hymenopteran insects, but far less is known about bird-pollinated flowers. Birds are hypothesized to exert different selective pressures on floral colour compared with hymenopterans because of differences in their visual systems. We measured the floral reflectance spectra of 206 Australian angiosperm species whose floral visitors are known from direct observation rather than inferred from floral characteristics. We quantified the match between these spectra and the hue discrimination abilities of hymenopteran and avian vision, and analysed these metrics in a phylogenetically informed comparison of flowers in different pollination groups. We show that bird-visited flowers and insect-visited flowers differ significantly from each other in the chromatic cues they provide, and that the differences are concentrated near wavelengths of optimal colour discrimination by whichever class of pollinator visits the flowers. Our results indicate that angiosperms have evolved the spectral signals most likely to reinforce their pollinators’ floral constancy (the tendency of individual pollinators to visit flowers of the same species) in communities of similarly coloured floral competitors.

色彩信号是推测型传粉综合征（pollination syndromes）中的核心线索。已有研究表明，多数花朵的反射光谱适配膜翅目昆虫（hymenopteran insects）独特的视觉辨别能力，但针对鸟媒传粉花朵的相关研究仍较为匮乏。由于视觉系统存在本质差异，相较于膜翅目昆虫，鸟类被认为会对花色施加差异化的选择压力。本研究针对206种澳大利亚被子植物（angiosperm）的花朵反射光谱开展了测量，这些植物的传粉类群已通过直接观测得以确认，而非仅通过花朵特征间接推断。我们量化了上述光谱与膜翅目、鸟类视觉的色调辨别能力之间的匹配度，并基于系统发育信息，对不同传粉类群的花朵进行了比较分析。研究结果显示，鸟媒花朵与虫媒花朵所提供的色彩线索存在显著差异，且此类差异集中于对应传粉类群的最优色彩辨别波长附近。本研究结果表明，在花色相似的竞争植物群落中，被子植物已演化出最有可能强化传粉者花朵恒定性（即单个传粉者仅访问同一物种花朵的倾向）的光谱信号。