1914] Chandler: Feathers of Circus hudsonius 351 



are spaced about twenty per millimeter. The distal barbules, 

 about thirty per millimeter, have short bases and extremely long, 

 slender tips, bearing a double series of well-developed spreading 

 barbieels. There are usually four moderate booklets of equal 

 size, and two strongly delimited ventral lobes. The nuclei of 

 both sets of barbules show very distinctly. 



The outer vane of the tail feathers differs decidedly from 

 the inner vane in its microscopic structure, and the proximal 

 barbules, twenty-three per millimeter, are of a rather peculiar 

 type (pi. 18, tig. 9). The total length is aboiat 0.9 millimeter, 

 the base being long, moderately stout, and tapering to the short 

 tip, there being no well-marked bend. The middle third of the 

 barbule is characterized by a series of moderately stout, blunt 

 projections, which are intermediate in form between barbieels 

 and ventral lobes, but seem more closely akin to the latter. The 

 nuclei of the barbules are large and conspicuous, this probably 

 being an indication of strong growth and abundant nutrition. 



The distal barbules, twenty-nine per millimeter, have a tip 

 considerably less than twice as long as the base, bearing long 

 brushlike barbieels on one side, and short, stout ones, or, in the 

 more basal ones, only rudiments, on the other — a very striking 

 difference from the long, slender barbules of the inner vane. 

 Their total length is scarcely 0.6 millimeters. 



Since the reetriees are such important feathers, and have 

 such a necessary and difficult function, they are, as we should 

 expect, strong, durable, and highly specialized to serve their 

 purpose in the economy of flight. While the shaft is strong 

 and stiff, it is not as heavily built as is the shaft of a primary, 

 since the stresses and strains acting upon the tail in steering or 

 balancing are not nearly so severe as those which the primaries 

 undergo during flight. Since in the tail a broad, resistant 

 surface is required, there is no incision of the feathers or rotation 

 of the vanes as in the primaries, and the tips of the feathers are 

 broadly roimded. The bend in the shafts of all the lateral 

 feathers to bring them to lie parallel with the middle ones, and 

 to prevent the tail from being permanently spread, is the most 

 interesting adaptation. 



