Figure 4.

Measured and predicted reflectance spectra of belly feathers of seven species of grackles and allies with iridescent plumage in direct perpendicular light. Predicted reflectance spectra were based on a three-beam model (model 1, figure 1) generated using the matrix thin-film optical method of Jellison (1993). All three interfaces of materials of different refractive indices within the barbule (air/keratin, keratin/melanin and melanin/keratin) were incorporated into model 1. Model 2 only incorporated the outer two interfaces and the thickness of the keratin layer. Model 3 only incorporated the air/keratin and melanin/keratin interfaces, and the thickness of the melanin layer. Model 4 only incorporated the inner two interfaces and the thickness of the melanin layer. Figures on the right-hand side are measured curves and predicted curves based on all four models, while figures on the left-hand side are measured curves and predicted curves based on the best-fitting model.