164 BULLETIN 107, UNITED STATES NATIONAL MUSEUM. 



usually near to some iceberg or ice pan, where tlie 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.— Ridgwuy^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 80-31, 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 apparentl}'^ the same 

 as in Cepphus gryUe. Dr. Witmer Stone (1900) publishes the fol- 

 lowing note on a series of birds from Point Barrow : 



Eight young (birds of the year), September 23, .January 11 (2), February 

 6, March 10, ALarch 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 mouk 

 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. 



