NON-INDIGENOUS BRITISH BIRDS. 309 



structures — heaps of half-rotten reeds, sedges, and other 

 marsh-loving plants, lined with finer and drier material, 

 such as sedge, leaves of the rushes and reeds, and dry- 

 grass. The behaviour of this Tern when its breeding 

 grounds are invaded is similar to that of allied species, 

 the birds rising in noisy crowds and fluttering to and fro 

 in an anxious manner, until the cause of their alarm 

 withdraws. 



Range of egg colouration and measurement : 

 The eggs of the Black Tern are three in number. They 

 vary in ground colour from clay colour to olive-brown or 

 olive-green, passing through almost every shade of buff, 

 blotched and spotted with reddish-brown and dark 

 blackish-brown, and with underlying markings of pale 

 brown and gray. As a rule the blotches are most 

 numerous on the major half of the egg^ and are often 

 confluent, but types occur in which the markings are 

 smaller and more evenly distributed over the entire 

 surface. Average measurement, I '4 inch in length by 

 ro inch in breadth. Incubation is performed by both 

 sexes, but the duration of the period is apparently 

 undetermined. 



Diagnostic characters : The eggs of the Black 

 Tern cannot be distinguished from those of the White- 

 winged Black Tern, and unless identified at the nest are 

 of no scientific value. It might be remarked, however, 

 that the Black Tern breeds in the Spanish peninsula, but 

 the White-winged Black Tern is not known to do so. 

 Locality in such cases is sufficient for identification. 



