REPRODUCTION AND SEX 517 



piece of rush, her ovary ripens, and she fixes oin one of the sug- 

 gestions of a nest, and begins to build rapidly. vShe is assisted by the 

 male, who has been watching her, and has also made some inde- 

 pendent experiments of his own. Yet even now the female's nest- 

 building is curiously punctuated, for after she has worked hard for 

 a short time, usually in the early part of the day, she turns aside 

 and attends to something else. Two or three days pass before the 

 finer strands are added to the nest and the lining is made. The last 

 of the five eggs is laid 19-22 days after the first attempt to build. 

 The male is still keen about his territory, but he is now for the most 

 part very silent, and sex-excitement is markedly waning both in 

 him and in her. He may even tolerate a rival male on his preserve. 



The fourth phase is marked by the common care for nest, eggs, 

 and young. With this the female is entirely preoccupied; for the 

 male it is a new attraction, but one subordinate to the claims of 

 the territory. "He finds food and distributes it amongst the young, 

 broods, cleans the nest, in fact does all that she does, and may do 

 it with even greater energy." Sexual behaviour has disappeared 

 for the time being, but it is interesting to notice that posturing still 

 persists, in the female at least, though the reaction with which it 

 was formerly correlated has disappeared. 



The courting bird is moved by constitutional impulses, heredi- 

 tarily engrained and expressed in a particular sequence, just like 

 other differentiations. The changes in the body as a whole influence 

 the gonads, and these liberate regulative hormones; and the 

 sequence is punctuated by external periodicities of temperature, 

 pressure, humidity, and the like, operating from without inwards. 

 Change of diet is probably in some cases another liberative stimulus, 

 but Mr. Howard does not say much about this. 



Let us look back on the four phases, well-marked in the Reed 

 Bunting and the Yellow Bunting and in some other birds which 

 select a "territory". 



(i) In the first phase the male behaves in five new ways — seeking 

 an appropriate headquarters or territory, occupying a conspicuous 

 position, indulging in exuberant song, sometimes experimenting 

 with nest-building, and fighting with rivals. He is in a condition 

 to behave sexually if stimulated. (2) In the second phase the 

 female arrives, but she is physiologically unready. The male's song 

 is popularly regarded as evoked by the female's presence; but 

 "instead of singing with renewed vigour, he gives it up and sings 

 but little— perhaps stops". The form of fighting among rivals changes, 

 and the male eagerly chases the female, who seems to enjoy it, 

 though not as yet fully responsive physiologically. (3) In the third 

 phase the female begins to posture or to posture in a new way. She 

 may even take the initiative and incite the male to pairing. This 



