ESSENTIAL CHARACTERS OF BIRDS 



table. The eyelashes and the bristles round the gape of birds 

 appear to be modified filo- plumes. In some birds, such as 

 herons, a peculiar kind of down is met with known as " powder 

 down/' because it breaks up on being touched into a fine powder. 

 This occurs in dense masses on the breast and thighs, or, as in 

 certain hawks and parrots, in isolated tufts, and is of unknown 

 function. 



The typical contour-feather is composed of a central axis, 

 divisible into (a) a calamus or quill (Fig. 18) and () a rhachis or 

 shaft (Fig. 18). Along the latter are ranged a series of long, slender, 

 flattened rods known as barbs (Fig. 18, B), which are held 

 together by a complicated system of hooks known as the bar- 

 bules (Fig. 18, Bs). Each barb bears two rows of barbules of quite 

 different structure, those pointing towards the tip of the feather 

 being provided with long, slender booklets (Fig. 19, A), while 

 those pointing towards the base of the 

 feather have the form of a series of 

 scrolls, and into the upper edges of 

 these scrolls the booklets are thrust 

 (Fig. 20). 



Thereby, as we have just indic- 

 ated, the whole series of separate 

 barbs is knit together, so to speak, so 

 as to form a continuous web, which 

 can only be broken with difficulty ; 

 and this web forms what is known 

 as the " vane " or " vexillum " of the 

 feather (Fig. 18, V). 



At the base of the shaft of 

 the contour- feathers of most birds 

 there will be found a second smaller 

 shaft, bearing a downy vane known 

 as the after-shaft (Fig. 21, A). What 



purpose it Serves, or why it should FIG. 21. A contour - feather of a 



be present, no one has yet been able pheasant, showing the large after- 

 to say. In "game-birds/' such as 



the pheasant, or in the common fowl, this after-shaft is quite large 

 (Fig. 21), while in perching-birds it is either reduced to a mere 



