256 SEA-BIRDS 



unmated tern can only discover the sex of another tern by testing its 

 reactions: if it submits to pecking it is female; if it fights it is a male. 

 The female, desiring to occupy a scrape, soon submits. After this 

 "ground recognition test," the two birds are mated for the season, 

 and recognise each other at some distance. But the dominance of 

 the male is kept up throughout the nesting season (Palmer). If an 

 intruder is a male the owner of the scrape must drive the visitor away, 

 or risk losing the territory. Even the arrival of the partner at the nest 

 immediately sets up this ceremony, or a modified form of it. The 

 bird at home is excited by the appearance of another from out of 

 the sky — excited and at first probably suspicious, in spite of visual 

 or vocal recognition — and so some form of greeting ceremony is 

 performed before all is quiet between the pair. These formalities are 

 instinctive, usually beginning with threat display which is then changed 

 to the opposite — invitatory or sexual movements, which serve as outlets 

 for the thwarted fighting instinct. These nest-visit ceremonies happen, 

 of course, in many other organisms ; in some birds, such as the herons, 

 they are quite elaborate and prolonged before the arriving bird is 

 able to relieve the bird in possession. 



Males, presumably unattached, sometimes visit occupied nests 

 and, encountering opposition, fight quite fiercely; and eggs may 

 be broken or rolled out of a nest as a result. In courtship the male 

 noddy invites the female, by nodding his head (hence perhaps the 

 name), to take the fish from his bill. This is characteristic of the sea- 

 terns. The female may accept and then return it to her mate; or, 

 more rarely, eat it. The pair may walk side by side in what the Marples 

 call "the parade." This is not unlike the action of the farmyard cock 

 soliciting the hen, and it has the same stimulatory function of promoting 

 hormonic activity and so releasing sexual desire. The cock pivots 

 round the hen, who pivots to face the performer. But the aggressive 

 pecking is over: as sexual excitement increases the head and tail are 

 raised at a sharp angle, the head being lowered again in a bowing 

 movement, displaying the hackles of the handsome nuptial cap. 

 Coition is initiated by the female crouching with her head down. 

 The cock may stand on the hen's back for some time, perhaps to be 

 interrupted by a neighbouring bird who has likewise been stimulated 

 by these attitudes. 



Nests are made rapidly and simply, by scraping in the sand or 

 shingle, or by adopting a depression in the rocks. In the sand the bird 

 falls forward on its breast and scratches backwards with its feet. 



