Read the passage and answer the question. Color in Bird Feathers The brilliant colors of bird feathers arise from two fundamentally different mechanisms, and distinguishing between them has occupied biologists for over a century. The first mechanism is pigmentation. Certain molecules, deposited in the feather as it grows, absorb some wavelengths of light and reflect others. Melanins, for example, produce blacks, browns, and dull yellows, and they have the additional benefit of strengthening the feather against wear. Carotenoids, which birds cannot manufacture themselves and must instead obtain from their diet, yield the vivid reds and oranges seen in species such as the cardinal and the flamingo. The second mechanism, known as structural coloration, does not depend on pigment at all. Instead, microscopic structures within the feather scatter light in ways that amplify particular wavelengths. The blue of a blue jay, for instance, is not produced by any blue pigment — indeed, no bird is known to manufacture a blue pigment. Rather, tiny air pockets and protein arrangements scatter short-wavelength blue light back to the observer while allowing other wavelengths to pass through or be absorbed by underlying melanin. This is why a blue feather, when crushed so that its internal structure is destroyed, loses its color entirely, whereas a crushed red feather retains its hue. The interplay of the two mechanisms can produce effects that neither could achieve alone. Iridescence — the shifting, metallic sheen of a hummingbird's throat or a peacock's tail — results when structural elements are layered so precisely that the color appears to change with the angle of view. Green, surprisingly, is rarely a pigment in birds; it usually emerges from the combination of structural blue with a yellow carotenoid layer. The evolutionary advantages of such displays remain debated, but most researchers agree they play a central role in attracting mates and signaling fitness. ────────── According to paragraph 3, the green coloration of birds usually results from