432 MR. O. SALVIN ON THE TAIL-FEATHERS OF MOMOTTJS. [Apr. I, 



perfectly formed spatules without any intervention on the part of 

 the birds themselves. 



In naturally denuded shafts the web gradually tapers towards the 

 bare part. If the character has been produced in the manner sug- 

 gested, we should expect such ultimately to be the case ; for those 

 webs nearest the untouched part would not always be removed with 

 the same certainty as those half-way towards the spatule. The 

 probability of their being removed would vary as the distance from 



Fig. 4. 



Tail of M. lessoni, with stems of cen- 

 tral rectrices partially denuded. 



Tail of Prionirhynchus platyrkynchus 

 with cenh-al rectrices asymmetrical. 



the base of the feather ; hence ultimately those nearer the base 

 would be less weakened than those further off, and a gradual dimi- 

 nution of the width of the feather towards the bare part be produced. 

 The same argument applies to the inner edge of the spatule. Further, 

 if this view is correct, the appearance of an abrupt passage of the 

 webbed feather to the naked rachis would of itself indicate the con- 

 tinuation of the nibbling-process on the part of the bird ; on the 

 other hand, the gradual passage to the bare rachis would indicate 

 that the habit was discontinued. 



This character, in the Motmots at least, is not peculiar to one sex 



