In human color vision, a combination of yellow and blue pigments creates various shades of green, as the reflected wavelengths from the surfaces stimulate cone photoreceptors in the retina to varying degrees. So to our eyes, the xanthophyll pigment in the Blue and Gold Macaw’s head feathers that reflect yellow wavelengths combined with the air spaces in those same feathers that reflect blue wavelengths of light cause our eyes to see the feathers as green in bright light.
But what does the bird see?
Birds have four types of cone photo-receptors (compared with the human three varieties), with the fourth type enabling them to detect much shorter wavelengths in the UV part of the spectrum.
Further, it appears that the coloration visible in the UV part of the spectrum may be of critical importance in signalling information about the fitness of an individual, just as the bright red coloration of a male cardinal signals its vigor and good health.
There’s a whole world of color out there that we are not able to see, and yet we make assumptions (and conclusions) about the various behaviors of birds and other animals, based on what we see. Makes you wonder what we’re missing.