182 DWIGHT 



black and white. The feather edgings of the back are grad- 

 ually lost down to the black area, the individual feathers thereby 

 becoming sagittate instead of rounded, while abrasion and fad- 

 ing remove the browns that conceal the white. I do not find 

 that the black area of any feather corresponds, except approxi- 

 mately, to the points where the barbules of adjacent barbs last 

 cross as figured by Chapman ('96) and Stone ('96, pp. 118- 

 119). I am inclined rather to believe that chemical disintegra- 

 tion proceeds faster in the less pigmented extremities of the 

 barbs which certainl}' are not provided with heavier barbules at 

 the point where the feather tips cease to break awa}'. Besides 

 wear, there is some renewal of feathers on the chin, throat and 

 sides of the head during February and March, as in man}^ other 

 species, but this perhaps scarcely deser\'es to be called a moult. 



5. Adult Winter Plumage acquired in Greenland by a com- 

 plete postnuptial moult late in Juh' and in August. The wings 

 and tail are usuall}' blacker than in first winter dress, the edg- 

 ings richer with less brown and more gra\% the tertiaries edged 

 with a deeper brown, the primar\' coverts wholly ^^•hite ; else- 

 where the brown is paler especially on the crown and jugular 

 band. 



6. Adult Nuptlal Plumage acquired chiefl}- b}- wear and 

 partly by moult as in the young bird. Plumage wholly black 

 and white. 



Female. — In juvenal plumage the female is similar to the 

 male, but with less white on the wings and tail, the greater 

 coverts brown, the primar}- coverts wholl\- dusk}% and the sec- 

 ondaries with dusky edgings. Subsequent plumages and moults 

 correspond to those of the male. The wings and tail are regu- 

 larly duller, and the white of the wing restricted and mixed with 

 dull black. The chief differential character is found in the feathers 

 of the head and nape which are dull brownish black basally. 

 In winter plumages this black is veiled with rich brown, but 

 wear produces a streaked appearance in nuptial plumages. The 

 jugular band is usually faint in females. The characters given 

 distinguish females in any plumage from males, whether adults 

 or young birds. 



