Fig. 4.
Ray-tracing simulations. a Simulation from FRED showing the trace of a ray that scatters multiple times between barbules of a super black feather. b Substantial variation in frequency of multiple scattering events among species predicts variation in structural absorption. c Measured reflectance is significantly negatively correlated with the proportion of reflected rays that scattered at least twice (linear regression: R 2 = 0.68, slope = −0.063, SE = 0.021, and P < 0.05)