i8 THE STORY OF THE BIRDS 



over almost the entire body, not only on the 

 bare places between the contour feather tracts 

 but amongst these feathers themselves. In other 

 birds they are confined to certain spots or are 

 developed in well-defined pterylae. These peculiar 

 feather growths are by no means common to all 

 birds, only appearing in certain groups. Some 

 of the most notable of these are the Herons and 

 Bitterns, the Parrots, the Tinamous, a few of the 

 Birds of Prey (Blue-winged Kite, Harriers), the 

 Frog-mouths (a group of Goatsuckers), and some 

 of the Rollers. The archaic character of these 

 powder-downs seems indicated by the fact that 

 only in a single genus (Artamus) of the extensive 

 and highly specialised group of Passeres are they 

 known to occur. Lastly we have to notice that 

 peculiar kind of feather which is known techni- 

 cally as a filoplume. A filoplume is composed of 

 a short barrel (calamus) terminating in a slender 

 hair-like shaft (rhachis), carrying few or no 

 barbs (rami), and forms what is popularly termed 

 an aftershaft, technically called an hyporhachis 

 (already alluded to in describing the various 

 parts of a feather). This filoplume starts from 

 the base of the quill and varies a good deal in the 

 degree of its development in the different avine 

 groups. It becomes specially small in the 



