Growth Bars in Feathers

TI growth bars in feathers are those elusive, alternate light and dark, normal angular markings across the web of many feathers and have the appearance of watered silk without any sheen. They have been referred to as the "ribbing" by Mascha (1905), the "fundamental bars" by Whitman (1907: 13) and by Riddle (1907: 165), also "fault bars" by Riddle, as "subordinate bars" by Glegg (1945: 301), "watered barring" by Forbush (1929: 322), and called just "feather bars" by Michener and Michener (1938: 149). Since they are caused by the normal development and indicate the rate of growth of feathers, the more descriptive title of "growth bars" is recommended. Their special characteristics for a species are general uniformity, visibility, width, and angulation. Chandler (1916) did not mention these growth bars in his extensive investigation but stated (p. 281), "There are no considerable variations in the feather structure of different species of the same genus." Since these characteristics are so individualistic, definite, and descriptive, the growth bars deserve more consideration than has been accorded them by ornithologists. These bars are most commonly seen in the wing and tail feathers, rarely in other tracts. While in most feathers they are not visible or only faintly, in some species they are very distinct in most feathers. In the feathers examined, they were as distinct in the juvenal as in the adult plumage. They are seen only by reflected light and best by biting the feather. They are not discernible optically by transmitted light, although shadow-graphs may reveal them, nor are their outlines made more distinct by magnification or by exposure to X-rays, infra-red, ultra-violet light, phosphorescent, or polarized light. Photographs and shadow-graphs are not as clear as visual impressions.