170 BULLETIN 107, UNITED STATES NATIONAL MUSEUM. 
white. The eggs are usually heavily spotted or boldly blotched with 
the darkest shades of brown or black; also with underlying spots 
and blotches of variout shades of drab, gray, lilac, or lavender, pro- 
ducing very pretty effects; many eggs are less boldly marked or. 
even finely speckled. The “meastinementa of 51 eggs in the United 
States National Museum average 60.5 by 41 millimeters; the eggs 
showing the four extremes measure 68.5 by 41, 64 by 43.5, 57 by 
39.5, and 60 by 38.5 millimeters. 
'Young.—Mr. W. Otto Emerson, in his notes sent to Major Ben- 
dire, gives the period of incubation as 21 days and says that both 
sexes incubate by turns, both of which statements are corroborated 
by others. He also says that the young are fed principally on small 
fish and do not leave the nesting site for the water until they are 
fully fledged. After the first few days:the young become very 
lively; they dislike daylight and, if exposed to it, will run away 
and hide in the remotest crevices; it is very difficult to catch them, 
much more so to photograph them, among the loose rocks where 
they live until big enough to fly. It is well that they are so secre- 
tive in their habits, for they have many enemies and protection is 
much needed at this critical time for the survival of the species. 
Plumages.—The young guillemot is hatched with a complete cov- 
ering of soft, thick down, “fuscous black” above, shading into 
“clove brown” below. Late in July or early in August the juvenal 
plumage begins to appear on the sides of the breast. The juvenal 
plumage is similar to the corresponding plumage in the black guille- 
mot, but it is usually more dusky both above and below, the dusky 
markings on the breast sometimes predominating. This plumage is 
worn through September, when a partial and gradual molt pro- 
duces the first winter plumage. Young birds in winter are darker 
than adults, especially on the upper parts, which are almost.wholly 
“fuscous black” without any white edgings; the throat and breast 
are mottled with dusky. A partial prenuptial molt in young birds 
takes place during March, April, and May, at which the black first 
nuptial plumage is partially acquired; but young birds may still 
be recognized by the wings, in which the white patches are mottled 
with black and which are not molted until the following summer. 
Beginning about the middle of August, a complete postnuptial molt 
occurs, and by the last of September, when the young bird is between 
14 and 15 months old, the adult winter plumage is acquired and 
old and young birds become indistinguishable. 
The adult winter plumage differs from the first winter in being 
almost wholly white below and in having the scapulars broadly 
edged with white, the feathers of the back narrowly edged with 
white and the pure white wing patches. Adults have a prenuptial 
molt involving all the contour feathers, at which the black plumage 
