332 Uinversitij of California Publications in Zoology [Vol. li 



and the down feathers, while in the Alaska specimen the contour 

 feathers have all been renewed, but the down feathers are still 

 growing in, a fact which indicates that the down feathers are 

 moulted slightly later than the contour feathers. The filoplumes 

 are renewed concomitantly with the definitive feathers to which 

 they are adherent. 



DOWN FEATHERS 

 Ordinary Down 

 As intimated above, the entire body of Circus is coveretl with 

 down feathers, this being a characteristic in which the diurnal 

 raptorial birds agree with the water birds, and differ from the 

 ground and perching birds, which, as adults, have down only in 

 the apteria. The down feathers of Circus are practically uniform 

 on all parts of the bird, though as a rule those occurring in the 

 midst of the contour feathers are smaller than those in the 

 apteria. The calamus is exceedingly short, considerably le.ss 

 than a millimeter long, and almost entirely buried in the skin. 

 As is usual with birds possessing aftershafts, the down feathers, 

 as well as the contour feathers, are furnished with aftershafts, 

 though in this case the two shafts, on opposite sides of the 

 umbilicus, are practically equivalent to each other. The division 

 between the two shafts at the umbilicus may best be looked upon 

 as a split in the quill, since the pith may always be seen pro- 

 jecting for some distance in front of the division into shafts. 

 As a rule, one shaft is somewhat longer and broader than the 

 other, but this is as likely to be on one side of the feather, 

 which projects at right angles to the body of the bird, as on the 

 other, and it is not likely that this difference in size should be 

 looked upon as significant of a differentiation of shaft from 

 aftershaft. At the base the shafts of the down feathers are as 

 broad as the diameter of the calamus, i.e., aboiit 0.20 to 0.25 

 millimeters, but it decreases rapidly with each pair of barbs 

 given off and entirely breaks up into barbs in from 3 to 7 milli- 

 meters. The barbs are exceedingly numerous, being about twelve 

 per millimeter on each side, or over six times as many as on an 

 ordinary flight feather or covert. The barbs are extremely long 



