SOCIAL AND SEXUAL BEHAVIOUR I73 



The opening of wings alerts a neighbour or partner; in the herring- 

 gull this action releases the attack drive, and it may stab at its mate's 

 wings, or even its own. Even when recognition takes place at a distance 

 (gulls and puffins can recognise their mates by sight ten metres away, 

 either in the air or on the ground) display takes place as the pair come 

 together. Often the untoward movement of one of the partners as the pair 

 sit or stand near the nest will start the other, or neighbours, displaying 

 or calling or both. Any movement within the colony is stimulating. 

 This stimulation, relating as it does to numbers, brings us to 

 consider the relations of the individual, and the pair to the breeding 

 flock, and what advantages there are in social nesting. Perhaps the 

 first thing to note is what determines the density of the breeding flock. 

 In spite of the availability of apparently suitable territory near by, 

 the social sea-birds prefer to crowd together in a specific and fixed 

 density pattern. As examples, gannets, increasing from 60 to 16,000 

 breeding adults in less than one hundred years at Grassholm, Wales, 

 have never varied the positioning of their nests one yard apart, and 

 today they occupy only two of the twenty-two acres of ground suitable 

 for expansion; and guillemots, which formerly occupied completely 

 a wide ledge two or three hundred yards long on the neighbouring 

 island of Skomer, as they decreased in recent years, instead of spreading 

 out thinly over the ancient breeding ledge continued to crowd together, 

 leaving large gaps which have become grown over with the nitrogen- 

 loving chickweed. The innate sociality of sea-birds is such that a 

 diminishing colony contracts towards the centre in the same fixed 

 pattern that an increasing colony expands; and the pattern is deter- 

 mined by the minimum territorial requirements of the breeding 

 pair — in the gannet a three feet radius from the nest-centre, in the 

 guillemot about twelve inches between each brooding bird. The 

 attraction towards the centre of population (evidence of sociality 

 in its highest form) is not unique to sea-birds ; it is a protective device, 

 common to many social animals. The breeding flock is composed of 

 a core of old adults in the best positions at the centre, surrounded by 

 a bulk of middle-aged members, and an outer skin of inexperienced 

 young birds. The greatest mortality occurs at the perimeters where 

 the young birds, detached, inexperienced, often wandering solitarily 

 and far on migrations, are most exposed to outside dangers. Until 

 they return and begin moving in towards the centre, they do not 

 become units in the main flock; but once they have re-established 

 themselves in the flock they will continue year by year moving towards 



