THE GULLS 23I 



But this is not the case, as we shall demonstrate later. Billing and mock- 

 or real feeding often precedes coition, which is conducted with much 

 wing-flapping on the part of the male, as he maintains position, utter- 

 ing a special loud deep note. Occasionally homosexual behaviour has 

 been recorded (Noble & Wurm, 1943; Goethe, 1937). 



In mild western districts, where, as at Skokholm, pairing may begin 

 even before the new year, mated herring-gulls may temporarily desert 

 the breeding ground, and fly together to the feeding ground. But as 

 spring approaches they are reluctant to leave the nest-site entirely, 

 and once nest-building begins one or other of the pair remains at or 

 near the focal point of the season's activities, especially in large colonies, 

 where stimulation and competition is greater. This stimulation, as 

 we have seen, may come from other species nesting close by. On 

 many islands and some cliflfs two or three species of gull may nest 

 together, and occasionally there is interbreeding (as between herring- 

 gulls and lesser blackbacks) . But if there is stimulation between related 

 species, there is no very visible interspecific competition. On Skok- 

 holm and Skomer, for instance, great blackbacks and herring-gulls 

 share the same plateau with much larger numbers of lesser black- 

 backs, but each species selects a slightly diflferent ecological niche 

 within the gullery. Thus the first two species are semi-resident, and 

 lay their eggs at the end of April or the beginning of May, always 

 selecting an outcrop of rock or a large stone against which the nest 

 is constructed; the great blackback dominating the higher outcrops, 

 the herring-gulls making use of rock- and loose-stone sites (which 

 are available in plenty) on the lower terraces of the outcrops not 

 occupied by the larger gull. The lesser blackbacks, arriving later 

 (in February and March), and laying their eggs from 10 May onwards, 

 use the ground between the outcrops, ground unencumbered except 

 with low vegetation of wind-blown bracken, heather, Aiolinia, etc. On 

 Skokholm there is probably enough room for all three; changes occur 

 in the numbers of each species, but the total gull population tends to 

 be more or less stable, and is kept from any great increase by the regular 

 collection of eggs throughout May and June by lighthouse-keepers 

 and others. 



The figures in the table (p. 232) are interesting as showing population 

 trends. The great blackback is known to be an increasing species in 

 the British Isles; at Skokholm the increase must come from outside 

 sources, since here this gull has been prevented from rearing more than 

 a very few young, by regular egg-collecting. Herring-gulls are able 



