1909] Beebe: Tail Feathers of the Motmot. 145 



tainty .... another shaft was found to be bare down to its 

 point of attachment by the side of the oil-gland; near the base 

 alone some corneous matter of uncertain determination, but per- 

 haps feather-material, was adhering to it. 



"These investigations tend to prove that no web at all is 

 produced with long-shafted rackets, but rackets of a lower stage 

 of development have imperfect or unattached webs which fall 

 off before the racket is fully exposed." 



Part IV. 



Now as to the relation in the Motmot, between the perma- 

 nently feathered and the ontogenetically defeathered areas of the 

 two rectrices. The fact of the narrowing of the webs has al- 

 ready been mentioned. In a specimen of Momotus mexicanus 

 Swains, in my collection (No. 819) the narrowing of the vane 

 begins considerably above the area of subsequent denudation. 

 At its narrowest part the constricted area, compared with the 

 width of the normal vane, is as 3 : 4. 



Microscopic examination yields certain results of value in 

 connection with my experiments. The full-grown right central 

 rectrice of one of my specimens, Momotus mexicanus (No. 819) 

 is untrimmed except for fourteen barbs on the inner web, the 

 absence of these leaving the shaft bare for a distance of 11 mil- 

 limeters. Now the racket in this species, as shown by several 

 other trimmed specimens, is about an inch and a quarter in length 

 and holding this untrimmed feather up against the light one can 

 perceive at once what will be the ultimate area of denudation. 

 Elsewhere the vane is dense and compact close up to the shaft, 

 but in this area the basal portion of the barbs is bare, allowing 

 the light to pass through, distinctly near the center of ultimate 

 denudation and graduating down to the normal condition of the 

 feather at the limits of the area. 



The rectrices of Momotus lessoni being larger and coarser 

 in every way, show this condition even more distinctly, especially 

 where a single stray barb has escaped the beak-trimming process. 

 Even in a fully trimmed feather, a certain amount of imperfec- 

 tion of the basal portions of the barbs is visible for some dis- 

 tance at each end, showing that the actual area of weakened 

 feather structure is greater than the area trimmed by the bird. 

 The result of the bird's preening is apparently controlled by the 

 weakness and ease of fracture of the barbs rather than by prox- 

 imity of other feathers or of visual estimate of the size of the 

 racket. 



