232 SEA-BIRDS 



Census of Gulls at Skokholm {pairs breeding) 



1929 1939 1949 



Herring-gull 

 Lesser blackback 

 Great blackback 



Total pairs .. 984 1160 994 



(taken from Letters from Skokholm (Lockley 1947) and 1949 Skokholm Report) 



to rear proportionately more young per pair at Skokholm because 

 many nest on inaccessible parts of the cliffs. The lesser blackbacks, 

 nesting principally on the plateau of the island, suffer most from egg- 

 collecting. It is probable that the gulls established their large main 

 colonies during the years 1890 to 1920 when the island lay more or 

 less abandoned and unfarmed (it was a prosperous farm one hundred 

 years ago), and before the lighthouse was placed in commission. 805 

 pairs in 1939 was a peak figure, and there has been a rapid decline since, 

 groups and little colonies disappearing from inland sites, while more 

 herring-gulls appear to have come inland to breed. But many factors 

 must be taken into account in trying to arrive at the true reasons for 

 the alterations which occur in sea-bird populations, especially in 

 studying the dynamics of the gregarious and migratory lesser black- 

 back, which is known to shift its breeding-grounds occasionally. 



At Hirsholmene in the Kattegat, Salomonsen ( 1947) found that 

 there was a similar, and even more, competitive sequence in the place- 

 ment of nests in three gull species nesting on the island. The black- 

 headed gulls formed the nucleus of the colony by laying eggs first — • 

 from 25 April onwards. A few common gulls, laying on i May, 

 placed their nests in a ring around the closed territory of the black- 

 heads. The smaller numbers of herring-gulls were also forced to breed 

 on this perimeter. The arrival of the Sandwich terns, which lay on 

 6 May, caused an upheaval due to the aggressiveness of the terns, 

 which forced their way into the centre of the black-headed gulls, 

 destroying eggs and driving the gulls away. Salomonsen suggests that 

 the attachment of the Sandwich terns and the common gulls to the 

 black-head colony may be due to the aggressive behaviour of the black- 

 heads in successfully driving away crow, hawk, fox, stoat, etc.: 

 also that the nesting drive of the terns is released by the sight of the 

 breeding colony of black-headed gulls. Other species of terns and gulls 

 are known to associate in breeding colonies; thus Sabine's gull 



