Color in a birds world

We have so far considered in some detail the structure of the bird's eye, and have shown how this structure has become adapted, during long evolutionary periods, to fulfil the requirements of sharp vision in the case of diurnal birds, and of maximum sensitivity to light in the case of nocturnal birds. But we know that our own eyes do more than pick out the shape and size of objects, or judge their distance away from us. Our eyes tell us also that different objects reflect the white light falling on them in different ways, and that consequently we recognize that they have color. It is pertinent therefore to ask the question, " Do birds see colors as we do ? " We can seek an answer to the question in two ways, firstly by considering the physiological structure of the bird's eye to see if it contains the prerequisites of color detection, and secondly by direct experimentation on the bird itself, by studying the reactions and choice of a bird when it is presented with a sequence of colors under controlled conditions. It can be stated at once that all the evidence points to diurnal birds possessing in varying degree, a fairly wide range of color vision, but although the experimental technique of investigators into the fascinating problem of color vision in animals has recently been greatly improved, it will be appreciated that the difficulties are extremely great. Several pitfalls have to be avoided, not the least of which is the assumption that because certain highly colored parts of the bird's body have sexual, or survival value, that therefore the bird itself must of necessity, be able to see and appreciate those colors This attitude arose soon after Darwin's theory of sexual selection had been propounded, when the habit grew up of assuming that since natural coloration generally had considerable utility from a sexual or protective point of view, that therefore those animals possessing such colors must of necessity also possess...