164 BULLETIN 107, UNITED STATES NATIONAL MUSEUM. 
usually near to some iceberg or ice pan, where the food is apparently more 
abundant, or where perhaps the water is quieter, at least on the lee side, or 
perhaps both. The young birds can not, or at least do not, remain submerged 
so long when diving at alarm. 
Plumages.—Ridgway’s Manual (1887) gives the description of 
the downy young as “uniform sooty blackish, paler and more gray- 
ish below.” On account of the late dates at which the young are 
hatched, they are often not fully fledged and ready to leave the 
nests until September 1 or later. According to Messrs. Thayer and 
Bangs (1914), Mr. John Koren visited a breeding place of this 
species at Cape Kibera Island, east Siberia, August 30-31, 1912, “ at 
which time all of the young birds were still in the nests. On Sep- 
tember 10 of the previous year, however, there were no guillemots to 
be seen at this same place, both young and adults evidently having 
left by that date. At Cape Irkaipig, September 6, 1911, a few 
birds were observed still feeding their young on the bluffs.” 
The juvenal and first winter plumages are apparently the same 
as in Cepphus grylle. Dr. Witmer Stone (1900) publishes the fol- 
lowing note on a series of birds from Point Barrow: 
Hight young (birds of the year), September 23, January 11 (2), February 
6, March 10, March 24, March 28, March 30, exhibit much variation in the 
amount of black on the head and black spots on wing coverts. All have narrow 
black tips to white feathers of the abdomen. None of these birds show any 
trace of the spring molt, which was well under way in the adults at the time 
that most of these were taken. 
Probably young birds do not acquire the full black nuptial 
plumage until the second spring, but at the first postnuptial moult 
they assume the adult winter plumage. Young birds are always 
darker or show more black mottling during the fall and winter than 
adults. They also have the mottled speculum. 
Adults have a prenuptial molt which is nearly complete, involving 
everything but the wings, which begins in March. They have a 
complete postnuptial molt, beginning about the middle of August 
and lasting a month or more; during this molt the wing feathers are 
shed almost simultaneously, rendering the bird flightless. The 
adult winter plumage is similar to that of Cepphus grylle, but it is 
much whiter. Adult Mandt’s guillemots can always be distin- 
guished from black guillemots by the white bases of the greater 
wing coverts and by the slenderer bills; in young birds this distinc- 
tion is not so well marked, but young Mandt’s guillemots have much 
less dusky at the bases of these feathers than young black guille- 
mots, where it occupies not only the basal half of each feather but the 
whole of the inner web nearly or quite to the tip. 
Food.—The food of Mandt’s guillemot seems to consist mainly of 
small fishes, crustaceans, and other soft-bodied sea animals. 
