212 THE SHELDUCK AND SURFACE-FEEDING DUCKS 



possibility of having been induced by adaptation. We refer to the 

 lower, or syringeal, end of the windpipe. This, and in the males only, 

 displays a curious, roughly spherical bony chamber, as a rule on one 

 side of the syrinx only. But in each species this bulla has a character 

 of its own, as distinctive as the plumage. Only in the sheldrake is a 

 symmetrically disposed double-bulla present. 



What purpose do these curious inflations of the syrinx serve ? 

 Apparently they are vocal organs, resonators. If this is so, one 

 would have expected the males to display the more powerful voice, 

 but the opposite is actually the case ; only in one or two species do 

 the males seem to utter any very peculiar sound, such as might be 

 attributed to the bulla. It may be, of course, that closer observation 

 will show that the males are more vociferous than was supposed, or 

 that the peculiar notes characteristic of the males of the several species 

 have a greater carrying power than we have hitherto imagined. 



When we turn to the " diving Ducks," we find this syringeal bulla 

 assuming still stranger forms. In the Eider-ducks alone does it bear 

 any likeness to that of the surface-feeding species. The Scoters form 

 a group apart, to be considered presently. In the rest this chamber 

 has become enormously enlarged, and its transformation seems to 

 have put a greater strain on the bone secretions concerned with its 

 formation. At any rate, the walls of the chamber are pierced by 

 large vacuities, across which are stretched exceedingly delicate 

 membranes. Here again we must regard the modification as con- 

 cerned with voice-production and not with the diving habits, since 

 only the males are affected. In the goosander alone does the female 

 show any tracheal modification. This takes the form of a fusiform 

 enlargement of the windpipe itself, and in the male there are two 

 such enlargements, which can be felt by drawing the fingers down the 

 neck of the living bird. The male merganser resembles the female 

 goosander in this respect, while the female merganser displays no 

 such contrivance. 



In the Scoters these syringeal bullae are wanting. In the common 



