NON-INDIGENOUS BRITISH BIRDS. 291 



Range of egg colouration and measurement: 

 The Qggs of the Great Black-headed Gull are three in 

 number. They are pale buff or drab in ground colour, 

 blotched, spotted, and streaked with various shades of 

 reddish-brown, sometimes approaching black in intensity, 

 and with underlying markings of paler brown and gray. 

 Average measurement, 3'0 inches in length, by 2"0 

 inches in breadth. The duration of the period of 

 incubation is unknown, as is also which sex performs 

 the duty. 



Diagnostic characters : The pale ground colour, 

 small and dark markings, and large size of the eggs of 

 this Gull prevent them from being readily confused 

 with those of any other species breeding in the same 

 area. 



Family LARID.-E. Genus Larus. 



Sub-family LARINrE. 



MEDITERRANEAN BLACK-HEADED 

 GULL. 



Larus melanocephalus, Natterer, 



(British : Very rare abnormal winter migrant.) 



Single Brooded. Laying season, May. 



Breeding area : South-western PaLxarctic recrion. 

 The breciding grounds of the Mediterranean Black- 

 headed Gull are most imperfectly known. This bird 

 may breed on the Atlantic coasts of Southern France, 

 and certainly docs so on those of South-western Spain — 

 in the delta of the Guadalquivir. It is also presumed 

 to breed in various parts of the basin of the Mediterranean, 



