NON-INDIGENOUS BRITISH BIRDS. 311 



rotten sedges, reeds, and equisetinns, and lined with bits 

 of dry reeds, leaves of the rushes, or coarse grass. 

 When disturbed at its breeding places this Tern be- 

 comes very anxious and noisy, rising in fluttering 

 crowds, and keeping in the air until left in peace. 



Range of egg colouration and measurement : 

 The eggs of the White-winged Black Tern are three in 

 number, but four are said to be laid in rare instances. 

 I have never seen a clutch of this latter number, but 

 have taken several clutches of four belonging to the 

 Lesser Tern. They cover precisely the same range of 

 colour variation as those of the preceding species, 

 rendering a detailed description unnecessary. Average 

 measurement, 1*4 inch in length by I'O inch in breadth. 

 Incubation is performed by both sexes, but the duration 

 of the period is apparently unknown. 



Diagnostic characters : The eggs of the White- 

 winged Black Tern cannot be distinguished from those 

 of the Black Tern, and therefore, to be of any scientific 

 value, must be carefully identified at the nest. 



Family ALCID^. Genus Uria. 



BRUNNICH'S GUILLEMOT. 



Uria troile brunnichi, Sabine. 



(British : Possibly breeds : Very rare nomadic autumn migrant.) 



Single Brooded generally. Laying season, June. 



Breeding area : North Atlantic and Arctic Ocean 

 basins. Brunnich's Guillemot breeds probably from the 

 Liakoff Islands westwards to Nova Zembla, Spitzbergen, 

 Franz-Josef Land, Grimsey Island north of Iceland, and 

 Greenland, north of about lat. 64°. 



