122 LIFE HISTORIES OF NORTH AMERICAN DIVING BIRDS 



dull dirty white with occasional stains. The measurement of 30 

 eggs, in various collections, average 54.2 by 37.9 millimeters; the 

 eggs showing the four extremes measure 60 by 41, 5, 59 by 42.5, 50 

 by 38.5, and 55 by 32.5 millimeters. 



Young. Incubation is apparently performed by both sexes, but its 

 duration is not known. Both parents also assist in the feeding and 

 care of the young, which remains hidden in the nesting cavity until it 

 is fully fledged and able to fly, about the last of August, or later. 



Plumage. The color of the downy young varies from "clove 

 brown" to "hair brown" above and from "hair brown" to "light 

 drab" below. When nearly grown the dark plumage begins to ap- 

 pear on the back and wings; the gray breast plumage appears next 

 and the natal down disappears last on the neck, chest, sides, rump, 

 and crissum, when the young bird is fully grown. 



The first winter plumage, which succeeds the natal down, is simi- 

 lar to that of the adult, dusky above the gray below; but the crest 

 and auricular plumes are entirely lacking and the bill is very small 

 and simple. This is the plumage which was described by Pallas as 

 Uria dubia. This plumage is worn during the winter until the par- 

 tial prenuptial molt in the spring, when the peculiar adornments 

 of the head are assumed and young birds become practically indis- 

 tinguishable from adults. 



The seasonal molts of the adult consist of a complete postnuptial 

 molt in August and September and a partial prenuptial molt in the 

 late winter or early spring, involving mainly the head and neck. 

 The conspicuous seasonal change is wholly in the head, which is so 

 striking as to have induced Pallas to describe the winter adult, as a 

 distinct species, under the name Alca tetracula. Fall adults can be 

 distinguished from young birds by having larger bills; the frontal 

 crests are also present, but are much smaller and less conspicuous 

 than in the spring; and the white auricular plumes are generally 

 more or less in evidence. The most striking seasonal change is in the 

 bill; in the spring and summer this becomes much swollen and very 

 grotesque in shape; its color is a brilliant "orange chrome," yellow- 

 ish horn-colored at the tip. Doctor Coues (1903) describes the curi- 

 ous combination of parts, which make up the bill, as follows: 



A nasal plate, filling nasal fossa, separate from its fellow of opposite side; 

 a subnasal strip prolonged on cutting edge of upper mandible backward from 

 nostrils; a semicircular plate at base of upper mandible over angle of mouth; a 

 large shoe encasing posterior part of under mandible the latter single, the 

 other three pieces in pairs, making seven in all which are molted. 



During the progress of the postnuptial molt, in September, these 

 deciduous horny plates are shed, after which the bill shrinks to its 

 winter proportions, the smaller dark-colored bill giving the bird an 

 entirely different appearance. 



