xvii. 7 DISPLAY 499 



building, incubation, and caring for the young necessitate a particular 

 state of excitability that must last a long time, and the display serves 

 to provide this condition in both birds. The reproductive process may 

 be interrupted at any time if the stimuli are inadequate. Thus ovula- 



Fig. 303. Incidents in the courtship of the great crested grebe. 1. Mutual head 

 shaking. 2. The female is displaying before the male who has dived and shoots out 

 of the water in front of her. 3 and 4. Further views of the male rising from the 

 water. 5. Both birds have dived and brought up weeds. (From Huxley, Proc. Zool. 

 Soc, 1914, by permission of the Zoological Society.) 



tion may depend upon courtship and copulation (pigeons) and eggs 

 are often deserted if the birds are interfered with in any way, or if 

 they fail to stimulate each other. 



The actual procedure of display is as varied as any other feature of 

 bird life. When the sexes are alike in colour and shape the performance 

 is usually mutual, as in the great crested grebes (Podiceps), which 



