42 



VERTEBRATE REPRODUCTIVE CYCLES 



land from which all other members of its species except 

 its mate are excluded, i^^ Then there is the question of 

 the nesting site itself, which is normally situated some- 

 where within this territory. This too must be acceptable, 

 and if it is not, as for instance in the case of the arctic 

 tern when the ground is waterlogged, then ovulation does 

 not occur and eggs cannot be laid. 122 Similarly, in India 

 and Egypt the kingfisher makes its tunnels into the 

 river banks as soon as the water level has fallen far 

 enough for it to do so, and not until then does ovulation 

 begin. 10 



Further there is the stimulus which the presence of 

 one bird exerts on another. The attachment of a male to 

 a female is often necessary before either of them can 

 reach full maturity. In this connexion it is known that an 

 isolated pigeon does not lay eggs, but if another pigeon, 

 male or female, is introduced, egg-laying begins at 

 once. This effect can even be achieved by putting a 

 mirror into the cage,'^^ ^nd thus it can be proved that the 

 stimulus is visual and not tactile, olfactory, or auditory. 

 In many species such mutual stimulation is carried to 

 extreme with the development of peculiar behaviour 

 patterns called sexual displays or courtship activities. 

 Particularly in birds, the failure or faulty performance of 

 these activities can prevent successful mating. 



Mutual stimulation is particularly highly developed in 

 colonially nesting birds. This was first demonstrated 

 by Darling who studied the herring gull and the lesser 

 black-backed gul^^ and it is probably also true of the 

 arctic tern.^^ For a long time it has been the habit of 

 these birds to live and nest in flocks so that nowadays 

 they are incapable of nesting successfully iw single 

 pairs. Unless a certain minimum number of nesting birds' 

 assemble in a group, the females are unable to lay eggs. 

 With larger and larger groups above this minimum the 

 egg-laying of the various pairs is earlier and more 

 closely synchronized. 



All these examples may be considered as instances of 



