TERNS AND SKIMMERS 253 



to arrive first are males; but sometimes it is a pair that is first seen, 

 or a flock. 



In general, terns are extremely capricious, nervous, and sociable; 

 these characteristics are shown in erratic movements, and in sudden 

 changing of a breeding site. A colony may be apparently fully 

 occupied, and often nests built up to the middle of May; then the 

 majority of the flock may suddenly leave the site and settle on a 

 new one, perhaps several miles away. In that case the few which have 

 started to breed may forsake their nests and eggs in order to join the 

 main colony, rather than continue in isolation. They are most successful 

 as breeders only when able to nest in large numbers close together. 

 Like other species of sea-birds terns seem to require this stimulation 

 of numbers. It is for this reason, it is suggested, that they will settle 

 down in an already advanced breeding colony of gulls or related 

 species, causing considerable disturbance and even driving away the 

 original gull colonists by their aggressive behaviour (see p. 232). 

 But A. O. Gross (1951) states that herring-gulls drive out terns (Massa- 

 chusetts) ; and Palmer (1941) observes that "when a gull species nests 

 in great numbers close to a tern colony, the terns usually go elsewhere 

 eventually." Sandwich terns were found by Salomonsen to dominate 

 black-headed gulls, and royal terns are known to boss black skim- 

 mers. Among marsh-terns, the black tern is bold and domineering 

 inland in North America (Bent). Other terns live socially in harmony: 

 as sooty and bridled terns; and common, arctic and roseate terns. 

 This harmony is generally due to differing ecological requirements. 

 Austin (1929) points out a possible ecological relationship in a 

 mixed ternery: the roseate tern has comparatively long legs, adapted 

 for walking about and nesting in fairly long grass; the common 

 tern with shorter legs prefers barer ground to walk upon; the arctic 

 tern with very short legs hardly walks at all and nests as a rule on 

 vegetationless ground. (In the Faeroes, however, the arctic tern, 

 with no competition from other species, nests freely in low annual 

 vegetation and grass.) Social stimulation and its significance in the 

 reproductive cycle of sea-birds has been discussed in chapter 7. 



Salomonsen (1943) found that large colonies of common terns start 

 laying and hatching earlier than smaller ones; but he found that 

 "the difference is only slight" — which might reasonably be expected 

 of terns, with their habit of arriving late, simultaneously, and ready 

 to breed. 



The breeding habits of terns have been intensively studied by 



