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Understanding how animals achieve simultaneous conspicuousness to intended receivers and crypsis to unintended receivers requires investigating the distribution, size, and spectral characteristics of color patches. Here we characterize plumage patterns of 40 rainforest bird species living in understory or canopy. Visual signals maximizing (or minimizing) detection are expected to differ between these contrasted light habitats, making rainforests appropriate to test hypotheses of color signal evolution. Using spectrometry and comparative analyses, we show that canopy and understory act as distinct selective regimes that strongly influence bird coloration. Birds reduce detectability by displaying countershaded patterns, by matching background color and contrast, and by reducing in size the most conspicuous patches. More intense on males than on females, selection for conspicuousness acts on large patches by increasing contrast on ventral parts likely to be seen by conspecifics. It also operates on small patches by focusing visual contrast on chest, head, and tail in understory and on wing and tail in canopy, by increasing local brightness contrast compared to general contrast in canopy, and by exploiting different wavelengths for contrast (short in canopy and long in understory). These results are of general interest to understanding the evolution of color patterns for all organisms living in contrasted light environments.

Crab-spiders (Thomisus onustus) positioned for hunting on flowers disguise themselves by assuming the same colour as the flower, a strategy that is assumed to fool both bird predators and insect prey. But although this mimicry is obvious to the human observer, it has never been examined with respect to different visual systems. Here we show that when female crab-spiders mimic different flower species, they are simultaneously cryptic in the colour-vision systems of both bird predators and hymenopteran prey.

Rainforests offer two contrasted light environments: a bright canopy rich in blue and UV and a dark understorey, rich in green and orange. Therefore, natural selection for crypsis should favour dark brown signals in understorey and bright green signals in canopy, whereas sexual selection for conspicuousness should favour bright yellow-red signals in understorey and dark blue and UV signals in canopy. Using spectrometry and comparative analyses, we examined the relationship between ambient light and colour signals in a bird community of French Guiana. It appears that brightness and hue are mostly naturally selected, while UV content of plumage is more likely sexually selected. At each height, both sexes present similar coloration but males display more conspicuous sexually selected patterns than females. These results show that ambient light drives the evolution of colour signals at community scale, and should be considered when studying signalling in other communities and light-contrasted ecosystems.