1914] Chandler: Feathers of Circus Itudsoiiius 3G3 



the upstroke of the wing they allow a free passage of air between 

 them, but during the downstroke the force of the wind closes 

 them together and an even, resistant surface is gained ; the 

 outer vanes are very strong and firm, but narrow and inflexible, 

 while the inner vanes are very broad and flexible, so that the 

 impact of the air during a downstroke of the wing is sufficient 

 to force the wide, under-lapping part of the inner vanes up 

 against the stiff outer vanes which act as braces, and the even 

 resistant surface already mentioned is produced; the terminal 

 portions of the first five primaries are abruptly narrowed so that 

 when the wing is spread the tips of these feathers are spread 

 out finger-like, and allow an even escape of air from under the 

 wing; and, finally, the increasing weakness and flexibility of the 

 innermost secondaries serves a function in the automatic adjust- 

 ment of the antero-posterior concavity of the wing for swift or 

 slow flight (see Headley, 1912). 



In the remiges and rectrices there is a minimum of downy 

 basal sti'ucture, this portion of the feather being reduced to a 

 very few downy barbs in the region of the superior umbilicus 

 and the lower portion of the outer vane. From the distribution 

 of dowTiy structure in the contour feathers in general, it would 

 seem that inadequate nutrition was the primary cau.se for its 

 growth rather than a slower rate of growth as suggested by 

 Eiddle (1908). Undoubtedly the portion of the feather germ 

 farthest from the shaft receives the poorest nutrition, and 

 it is significant that in all the barbs the terminal portions 

 which lie farthest from the shaft in the feather germ have a 

 tendency to become downy in nature. When the feather is 

 near completion, and the nutritive supply is reduced and finally 

 cut ofl'. two changes take place which it seems to me must be 

 correlated with the reduction of nutrition; namely, the change 

 from a differentiated to a downy t.ype of barbule, and a diminu- 

 tion of the rate of growth, the latter entailing a more frequent 

 repetition of parts, i.e., a production of a greater number of 

 barbs and barbules per unit of distance. This reduction in rate 

 of growth was taken by Riddle to be the primary cause of the 

 change in type of structure, but it seems more reasonable to 

 look upon both as results, independent of each other, of the 



