70 PROCEEDINGS OF THE ACADEMY OF 



parts pure white; the back, (and frequently the crown, and back of neck,) 

 bhxck, more or less variegated with white. Audubon figures this condition 

 very nearly. 



The change in spring — mostly occurring during April and May — is the reverse 

 of that already described as the autumnal moult. 



Fledgelings. — (Labrador, July, 1860, E. Coues, Mus. Smiths. No. — .) Length 

 about 6-00 ; bill -50 ; tarsus -60 ; middle toe and claw -90, etc. Wholly covered 

 with soft wool}' puffy down, fuliginous brownish black ; bill and feet brownish 

 black. 



Young, first plumage. — Traces of the down on various parts of the body ; the 

 bird probably just beginning to fly ; length about 10-00, wing 11-50 ; bill 1-00, 

 black; tarsus 1-25; reddish dusky, as also are the toes. U|)per parts plumbeous or 

 sooty black, scarcely varied with white. Mirror beginning to appear, as white 

 spotting on a blackish ground. Entire under parts white, thickly marbled, 

 rayed and undulated with light touches of dusky. 



This state tends to pass directly into a condition exceedingly similar to, if 

 not identical with, that of the adults in winter. But birds of the first winter 

 may, at least early in the season, be distinguished from old ones by a certain 

 •' feel " of the plumage, and a shorter, weaker bill, less developed as to its 

 ridges and angles. 



Accidental variations. — The foregoing descriptions apply to the various stages 

 of plumage, which are strictly normal in character, and which, though un- 

 ending in precise degree, and varying with almost every individual, merge in- 

 sensibly into each other. The species is, however, also very subject to acci- 

 dental and entirely abnormal variations. Of these, albinism is the most com^ 

 mon. (Spec, in Mus. Acad. Philada.) Entirely milk white, without a trace 

 of black ; bill and feet light colored ; ej'es probably pink in life. The oppo- 

 site condition of melanism is described by authors. This consists in the total 

 absence of white on the wings ; and is apparently of infrequent occurrence. 

 Both these conditions have been described and named as characterizing dis- 

 tinct species. In the latter, the bird must not be confounded with Uria carlo, 

 which is totally different. 



Dimensions. — Adult : Length, (average) 13.00 ; extent, (average) 22-50 ; 

 wing 5-50 to 6-25; tail 2-00, a little more or less ; bill along culmen 1-30; 

 along rictus 1-75 ; along gonys -65 ; depth at base -45, width -35 ; tarsus 1-25 ; 

 middle toe and claw r'75, outer do. slightly less, inner do 1-40. 



It may be of advantage to look closel}' into the formation of the white area 

 upon the wing of this species, to the end that its composition may be clearly 

 understood, and recognized as different from that which obtains in the allied 

 species, U. columha. The mirror upon the upper coverts varies to a degree, 

 and in a precisely similar way, in each species ; but when perfect constantly 

 presents a radical difference. 



When Uria grylle is observed flying, as is its wont, low over the water with 

 rapid beats of the wings, the eye receives the impression of a black bird, with 

 a large white circular spot on the wing. This spot is constantly in view, 

 and represents the retinal image resulting from the white spaces upon both the 

 upper and under surface of the wings blended together by the rapid motion of 

 the wings. Those who have observed Uria grylle in its native haunts will 

 appreciate the pertinence of this remark. Uria coluniba presents no such 

 peculiarity of appearance, there being no white upon the under surface of the 

 wings ; and the eye readily follows the movement of the small white space 

 upon the wings, as with the changing attitudes of the bird, it is now apparent, 

 now lost to view. 



In Uria grylle, the row of great coverts upon the secondary quills are basally 

 black, terminally white. The outermost are white for rather less than half 

 their length, and the white occupies chiefly the exterior webs. Nearer the 

 body they are white for more than half their length, and the white occupies 

 both webs of the feathers. 



[Jan. 



