Vol. XVII 
ee ALLEN, ‘ Aptosochromatism. 22T 
grasp upon fundamental principles, and as long as such observers 
expect to be taken seriously, they must not be surprised if they 
are called sharply to account.” . 
A careful study of Mr. Birtwell’s paper has convinced me that 
there is nothing very unusual about Mr. Birtwell’s caged Indigo 
Bunting. No ‘hypothesis’ is necessary to account for its moult- 
ing at the time it did, nor would Mr. Birtwell have thought any 
‘hypothesis’ necessary if he had known the simple facts that the 
species normally moults twice a year, and that the prenuptial 
moult is ordinarily more or less incomplete. Nor is there any- 
thing in the color changes described incompatible with the belief 
that the changes observed were wholly due to the normal shed- 
ding of the tips of the feathers, both in the new feathers and in 
the old ones. .The gradual wearing off of the tips of the feathers 
necessarily results in the exposure more and more of the previ- 
ously concealed blue basal portion of the feathers underneath 
them, which would result in “the apparent brightening of the 
blue portion of the feather, beginning evenly on each vane from 
the bottom,” as remarked by Mr. Birtwell. He further says: 
‘¢ When the band of tawny was reached, it appeared slowly to be 
absorbed until but faint tips of this color could be seen upon the 
ends of the larger barbs.” Here almost certainly Mr. Birtwell’s 
observations were in error, as he could easily have himself detected 
had he been familiar with the differences in structure between the 
fugacious buffy tip and the main body of the feather; although 
he gives it as his opinion that “in no case.were the barbs or bar- 
bules broken off sufficiently to account for the change.” ‘There 
is nothing to indicate that Mr. Birtwell’s bird was not a male of 
probably the second (possibly of the previous) year undergoing 
(1) the normal spring moult of the species and (2) gradually 
changing color, in case of both the old plumage and the new, by 
the usual wearing away of the fugacious brown feather tips and 
gradually exposing more and more of the previously concealed 
basal portions of the feathers, as occurs normally, to a greater or 
less extent, in hundreds of species of our birds, and so markedly 
in such species as the Snowflake, Bobolink, and many others that 
might be mentioned. 
Mr. J. Lewis Bonhote has also shown of late much interest in 
