“Every species of bird is possessed of a certain, not always definable, cast of countenance, peculiar to itself.  Although it undergoes changes necessary for marking the passions of the individual, its joy, its anger, its terror or despondency, still it remains the same specific look.  Hawks are perhaps more characteristically marked in this manner than birds of any other genus, being by nature intended for deeds of daring enterprise, and requiring a greater perfection of sight to enable them to distinguish their prey at great distances.  To most persons the family-look of particular species does not appear so striking as to the student of nature, who examines her productions in the haunts which she has allotted to them.  He perceives at a glance the differences of species, and when he has once bent his attention to an object, can distinguish it at distances which to the ordinary observer present merely a moving object, whether beast or bird.  When years of constant observation have elapsed, it becomes a pleasure to him to establish the differences that he has found to exist among the various species of a tribe, and to display to others whose opportunities have been more limited the fruits of his research.”

–J. J. Audubon, Ornithological Biography, I (1831), 364 [excerpted].