EXTERNAL PARTS OF BIRDS.— FEATHERS. 93 



"black portion of the feathers, so that the white spots are lost as neatly as if they had been 

 snipped away with scissors. It seems to be a rule, that heavy pigmentation tends to make a 

 feather more durable than it would othervAise be. Again, the identical primary of a Gull ac- 

 quires after it is full grown a greater extent of white or pearly web than it had before, by actual 

 absorption or decomposition of black pigment in a portion of the web, the bleaching being thus 

 progressive. Such changes could not go on in a dead feather; they are physiological processes, 

 or at any rate chemical processes, in living tissue, — not merely mechanical alterations due to 

 wear and tear of dead substance ; and they affect coloration of plumage far more profoundly 

 than is commonly recognized, as well as in a far greater number of cases than have been as- 

 cribed, as they should be, to aptosochromatism. It has been proven in the case of the red and 

 gray phases of our Screech Owl (Megascops asio) that aptosochromatism is effected by actual 

 alteration of pigmentation, without any loss of old or gain of new feathers. Erythrlsm and 

 melanism, and numerous other alterations of color, may be undergone by birds without any 

 moult ; such change may be brought about by a particular diet, and certainly this could not 

 occur if a grown feather were a dead feather, lacking all vascular connection with the fluids or 

 humors of the body. Such physiological or chemical processes as are concerned in depigmen- 

 tation and rcpigmentation of grown yet living feathers may be likened to the changes undergone 

 by chlorophyll in the leaves of plants which change from green to yellow or scarlet while they 

 still live, and do not lose vascular connection with the stem till they turn brown, wither, and 

 drop. Persons who pluck live geese understand this matter better than some ornithologists do ; 

 they resort to this cruel process because they can get a better pi-ice for feathers torn from the 

 living body of the poor bird, than for the identical feathers taken from the same goose dead, 

 because the former are more elastic and more durable. 



Aptosochromatism is thus primarily a physiological and chemical fact. But it extends to and 

 is directly connected with a certain mechanical process by which plumage may be profoundly 

 affected in coloration without loss or gain of any feathers. Now, if the student will refer back 

 to what I have said regarding color, he will recall the facts, that pigmentary colors are often 

 dependent upon the texture of feathers for their optical effect. For example, there are no blue 

 pigments, but plenty of birds are blue by objective structural coloration ; and any alteration in 

 texture or structure of a feather is liable to produce a change of color. In fact, this sort of ap- 

 tosochromatism is very common ; it consists in shedding certain parts of a feather which have 

 less vitality than the rest, and therefore break off and drop away before the whole feather dies 

 and follows suit. Not only barbicels and barbules may be thus moulted, without visibly alter- 

 ing the shape of the feather, though very likely with some change of objective structural color- 

 ation, and in cases of iridescence with entire change of subjective structural coloratiou ; but also 

 some of the barbs themselves may fall away from the rest, with great change in the figure of 

 the webs, and consequently great alteration in color of plumage if, as is usually the case, the 

 lost portion of the webs be differently pigmented from the part that remains. Few ornitholo- 

 gists seem to be aware of the prevalence of this sort of aptosochromatism as a factor in modifying 

 or entirely changing the coloration of birds. The male of our Bobolink, for example, acquires 

 his faultless black plumage by shedding the long yellowish tips of the feathers which just before 

 had veiled those portions of his wedding suit. The Snow Bunting, which has no spring moult, 

 passes to the pure black and white nuptial plumage by dropping the brown edges and ends of 

 black centred feathers; in this case, so much of each feather is lost that the shape changes from 

 a broadly rounded to a sharply pointed contour. In the related genus of Longspurs I have 

 found that certain uniformly glossy black areas result in like manner from loss of deciduous gray 

 or brown portions of the webs. I have above spoken of this kind of aptosochromatism as "a 

 certain mechanical process." It is mechanical in the sense of breakage and loss of parts of a 

 feather, but this is not due to actual abrasion or wear and tear, and would not occur if there 

 were no physiological process concerned ; for if the parts in question were not devitalized they 



