BinDS AND TBEIR FEATHERS 11 



Many a feather' has, besides what is above described, another 

 rhachis, on its lower side, called an aflershaft. This after- 

 shaft joins the scape at the umbilicus, and has on its sides 

 barbs and barbules about the same as those on the main rha- 

 chis. This part of 

 the feather, even 

 when present, is, 

 in all of our birds, 

 much smaller 

 than the main 

 vane. The figure 

 shows a feather 



from the back of the English sparrow, with an aftershaft, and, 

 at the right, the aftershaft separated from the feather. 



The description so far given is that of the usual feather, and, 

 if the aftershaft is present, of a complete feather. There are, 

 however, many modifications of these forms, concerning which 

 some knowledge is important. First, as has already been said, 

 the aftershaft is frequently wanting. Sometimes the barbs 

 are found on only one side of the rhachis ; this makes a one- 

 sided vane. Frequently the barbs are lacking on both sides, 

 thus changing the feather to a bristle,* as around the 

 mouth, nostrils, and eyelids of most birds. Some- i^KsJ~~ 

 times the barbs lack barbules on certain sections of »^*^^_: 

 their length, forming feathers with transparent por- 

 tions. Sometimes the barbs are so far apart that there can be 

 no locking of barbules, even when present ; this causes the for- 

 mation, in certain cases, of the most beautiful of plumes, as in 

 the " aigrette " of the herons during the breeding season. 



In review, it is well to recall the types of feathers spoken 

 of in the foregoing pages, and to notice examples of each, as 

 shown in the English sparrow. 



1. The typical feather, or pen feather, where the interlocking 

 of the barbs is complete, as in the great quills of the wing. 



2. The complete feather, where there is an aftershaft as well 

 as the main vane, as in the larger feathers of the back. 



