Many animals have the potential to discriminate nonspectral colors. For humans, purple is the clearest example of a nonspectral color. It is perceived when two color cone types in the retina (blue and red) with nonadjacent spectral sensitivity curves are predominantly stimulated. Purple is considered nonspectral because no monochromatic light (such as from a rainbow) can evoke this simultaneous stimulation. Except in primates and bees, few behavioral experiments have directly examined nonspectral color discrimination, and little is known about nonspectral color perception in animals with more than three types of color photoreceptors. Birds have four color cone types (compared to three in humans) and might perceive additional nonspectral colors such as UV+red and UV+green. Can birds discriminate nonspectral colors, and are these colors behaviorally and ecologically relevant? Here, using comprehensive behavioral experiments, we show that wild hummingbirds can discriminate a variety of nonspectral colors. We also show that hummingbirds, relative to humans, likely perceive a greater proportion of natural colors as nonspectral. Our analysis of plumage and plant spectra reveals many colors that would be perceived as nonspectral by birds but not by humans: Birds’ extra cone type allows them not just to see UV light but also to discriminate additional nonspectral colors. Our results support the idea that birds can distinguish colors throughout tetrachromatic color space and indicate that nonspectral color perception is vital for signaling and foraging. Since tetrachromacy appears to have evolved early in vertebrates, this capacity for rich nonspectral color perception is likely widespread.

许多动物有辨别非光谱颜色的潜力。对于人类来说，紫色是非光谱颜色最明显的例子。当视网膜中具有不相邻光谱灵敏度曲线的两种色锥类型（蓝色和红色）主要受到刺激时，就会被感知到。紫色被认为是非光谱的，因为没有单色光（例如来自彩虹的光）可以引起这种同时刺激。除了灵长类动物和蜜蜂之外，很少有行为实验直接检验非光谱颜色辨别力，并且对具有三种以上颜色感光器的动物的非光谱颜色感知知之甚少。鸟类有四种色锥类型（人类有三种），并且可能感知其他非光谱颜色，例如紫外线+红色和紫外线+绿色。鸟类能否辨别非光谱颜色，这些颜色与行为和生态相关吗？在这里，通过全面的行为实验，我们证明野生蜂鸟可以区分各种非光谱颜色。我们还表明，相对于人类，蜂鸟可能将更大比例的自然颜色视为非光谱。我们对羽毛和植物光谱的分析揭示了许多颜色，这些颜色会被鸟类感知为非光谱颜色，但不会被人类感知：鸟类的额外锥体类型使它们不仅能够看到紫外线，还能辨别其他非光谱颜色。我们的结果支持这样的观点，即鸟类可以在整个四色颜色空间中区分颜色，并表明非光谱颜色感知对于发出信号和觅食至关重要。由于四色视觉似乎在脊椎动物中很早就进化出来了，这种丰富的非光谱色彩感知能力可能很普遍。