SOCIAL AND SEXUAL BEHAVIOUR 1 75 



When in a flock a sentinel bird sees an enemy and flees, or flies to 

 attack it, the rest of the flock instantly heed the signal (a cry of warning, 

 a display of wing or tail pattern, or merely the opening of the wings, 

 or all of these signs together), and they too rise in the air. Thus a 

 mood of uniform alertness is elicited in every member of the flock at 

 the first notice of danger, and this is an advantage of numbers which 

 has obvious survival value. So swift is the bird's reaction to warning 

 and other signals that the flock may appear to act wdth complete 

 synchronisation; but the mood is not really instantaneous, it is acquired 

 by each bird individually copying another, and in the largest flocks 

 the mood is seen to pass through the birds in a wave. 



So-called synchronised flights also occur for no apparent reason, 

 initiated in the same way by a leader or dominant or nervous bird, 

 such as the "dreads" and panic flights of terns. Other combined 

 operations, such as the joy-wheels of puffins and other auks, the mass 

 diving of guillemots, the nocturnal circling flights of petrels, and the 

 summer assemblies of gulls and oyster-catchers in favourite spots 

 away from the nesting sites, seem to have a sexual significance. They 

 may be caused by a temporary re-assertion of the flocking instinct 

 (Tinbergen, 1951); their function seems to be that of sustaining a 

 high emotive tone, for they are set in motion by the most exuberant 

 individual in the flock, whose example and mood is so infectious that 

 it is rapidly mimicked by the whole community. Herring-gulls will 

 automatically preen in unison, copying a leader, as sub-consciously 

 as man may copy another man yawning. 



In a colony of sea-birds the most mature adults are the first to 

 take up territory and breed in the spring; their display and pre- 

 coitional activities set the pace for the less mature members of the 

 community. Their example acts as a stimulus, with psychological 

 and physiological effect on their neighbours; their first matings 

 promote the hormonic development and the ripening of ova in neigh- 

 bouring pairs. Their mood elicits a like mood in the others, and the 

 sexual rhythm of the whole colony is quickened as the emotive tone 

 rises. Occasionally, if their females are not ready, the stimulated males 

 may even attempt adulterous matings (gulls, puffins, albatrosses). 

 Usually however, the males are occupied in inducing their own mates 

 to copulate within the nest-site territory, and their display and 

 behaviour has the efifect of synchronising the female sexual rhythm 

 with theirs. All this intensive stimulation results in earlier and more 

 uniform breeding in the large flock. 



