The BeJiavior of Noddy and Sooty Terns. 207 



from one another by distances so regular that the unaided eye can with 

 difficulty distinguish inequalities in the spacing. 



The above reaction was discussed in this connection because of its bear- 

 ing upon the question of the presence of an " organic time-sense " in the 

 noddy. The birds (i?'s) often leave these chosen spots to go to the nest 

 and relieve the mate. The probable conclusion to be drawn is that organic 

 impulses furnish the stimulus leading to the return to the nest. If this be 

 the case, we should expect the functioning of such a time-sense to be as 

 inaccurate as our chart actually shows the case to be. 



Summing up this long section on the behavior of the brooding noddies, 

 we find : ( i ) that the presence of the egg brings about a change in the dis- 

 tribution of labor between the sexes; (2) the male no longer feeds the 

 female, but each sex separately obtains its food; (3) the tg^ is brooded 

 constantly day and night by both sexes, the male and female relieving each 

 other at intervals varying from 30 minutes to 5 hours, the average interval 

 being in the neighborhood of 2 hours ;' (4) the most significant general 

 reaction caused by the presence of the egg is the change in the disposition 

 of the birds. Before the egg appears, the birds are shy and leave the nest 

 at -the slightest disturbance ; after the egg is laid, the birds will defend the 

 nest against even human invaders. 



It is a little hard for the student absorbed in mammalian behavior alone 

 to understand the fixed character of all these responses — the relative lack of 

 any large store of latent adaptability and plasticity. These reactions preserve 

 the birds and have preserved them for ages in this favored environment. 

 There is no need for a larger repertoire of reactions. There is good material 

 here for the study of the acquisition of habits and the permanency of such 

 habits in an organism which is by nature already so largely adapted to its 

 environment. 



ACTIVITIES BY THE SOOTY AFTER THE EGG IS LAID. 



The sooty, like the noddy, as a rule lays one egg, although I counted 

 some 25 cases where a nest contained 2 eggs and, in one observed case, 2 

 birds were actually hatched and reared. The first egg was laid on May 7. 

 By May 15 thousands of eggs were present. Fisher- has the following to 

 say concerning the markings of the sooty egg: 



The ground-color is white or occasionally a cream buff. One type of marking 

 consists of deep burnt sienna and grayish vinaceous spots, with occasional nearly 

 black scrawls scattered rather evenly over the whole surface. These spots are I. 2, 

 and 3 mm. in diameter, with occasional larger and smaller ones. Another less prev- 

 alent variation consists of heavy, very deep burnt sienna blotches (s to 15 mm. in 



' It would be interesting to determine whether the birds relieve each other at 

 night, or brood the egg on alternate nights, or finally, whether one sex always re- 

 tains possession of the nest at night. 



" Op. cil., p. 780. 



