122 BULLETIN 107, UNITED STATES NATIONAL MUSEUM. 
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. 
Plumages—tThe 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 and 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 
eritirely different appearance. 
