348 University of California Publications in Zoology [Vol. 11 



While in general the coverts are not so highly developed and 

 specialized as are the remiges, nevertheless they are thoroughly 

 adapted for their function, and show numerous adaptive adjust- 

 ments which command admiration. The sigmoid curve of the 

 quills of the greater upper coverts to bring them to lie between 

 two remiges, the increased depth of the shaft to make them more 

 resistant to vertical stresses, and the increasing weakness and 

 flexibility of the shafts and vanes proximally to parallel the 

 same condition in the remiges, are niceties of adjustment which 

 should not be overloked. The greater under coverts are especi- 

 ally fitted for their function of covering the bare calami of the 

 remiges by their very short calami and smooth flat form. Their 

 reversed position, i.e., with the umbilicus exposed, is not an 

 adaptive, but an ancestral character. The reversed overlap of 

 all the upper coverts except the greater ones is difficult of 

 explanation, unless it is possible that the wind, coming forcibly 

 from the distal side of the feathers during flight, might be more 

 likely to ruffle the feathers if they had the same overlap as the 

 remiges. In these feathers, which are not subjected to the 

 severe strains and stresses imposed upon the remiges, the minute 

 structure is much simpler, the downy portion of the feather is 

 increased, and the shafts are weaker. Nature is lavish only 

 when there is need for it. The humerals, which perform an 

 important function in bridging over the gap between the inner- 

 most .secondaries and the body of the bird, are again highly 

 developed and specialized, being almost precisely the same in 

 form as the innermost secondaries which they follow. 



ALULA 



The only other feathers of the wing which merit special 

 description are the thumb feathers or alula, consisting of four 

 feathers at the base of the outermost primaries. Their form is 

 a good example of structural adaptation. Obviously, to have the 

 edge of the wing which cuts the air formed by the edges of more 

 than one feather would result in much rufHing and loss of energy. 

 This difficulty is avoided on the distal half of the wing by the 



