Like all birds, raptors have not merely two (as we do) but three types of photoreceptors in the eye. Because of this, birds are thought to be able to experience colors that humans cannot even describe. Their retinas, unlike ours, contain few blood vessels. Instead, a thin, folded tissue called pectin, unique to birds, brings blood and nutrients to the eye without casting shadows or scattering light in the eye as blood vessels do.
