NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 81 



ited. There can be little doubt that this second bird was the female, that th(> 

 stimulus in the case of the male was purely visual and tactual, while in the case 

 of the female the act of laying was necessary before the brooding phase could 

 appear. 



It is generally stated by the natives of the region that the sooty will lay a 

 second egg if the first is taken from the nest. I was unable to verify this. 

 The eggs were taken from 10 nests and in no case was a second egg deposited; 

 the nests were all abandoned after a few days. However, none of these nests 

 was less than a week old, and it is possible that the loss of the egg at an early 

 period may serve as a stimulus to further oviposition. 



An attempt was made to find out how long incubation might be interrupted 

 by removal of the egg without leading to the abandonment of the nest. Eggs 

 were taken from a number of nests and returned at regular intervals, from 6 

 hours to 4 days. Brooding activities were resumed by the birds after 4 days. 

 The experiment had to be interrupted here because of stormy weather.* 

 From rough observations, however, I believe that after a week's interruption 

 the birds will not resume incubation. The time is certainly less than the 

 time required for the loss of the brooding instinct of birds in captivity, t as 

 might be expected from the greater opportunity which free binls have to 

 form new habits toward the colony. 



After the hatching of the egg the parental instinct seems to be even more 

 dependent upon the stimulus of the chick in the nest. In some cases nests 

 from which the chicks were removed seemed to be wholly abandoned after 

 48 hours. 



The second marked change in the behavior of the nesting birds occurs at the 

 hatching of the eggs. The incubation period of the noddies is given by Wat- 

 son as 32 to 35 days, that of the sooties as 26 days. I matle attempts to shorten 

 and to lengthen the time required for incubation in order to test the extent 

 to which iiatra-organic factors are concerned in the changes from incubation 

 to brooding activities. The experiments are very unsatisfactory, chieHy 

 l:)ecause the age of very few nests was known. 



Substitution of .young chicks for noddy eggs of any age was not followed by 

 any marked change in the behavior of the adult birds. They were never 

 seen to attack the chicks, but I could not determine how soon such chicks 

 are fed. It seems certain, however, that the incubation period of the noddy 

 may be shortened by at least one-fourth without serious interference from an 

 intra-organic rhythm. 



Attempts to shorten the incubation time of the sooties have not been cari-ied 

 out fully, but there is more evidence that the length of the incubation period 

 is detei'mined, to some extent, by internal factors. Parents whose eggs an; 

 less than two weeks old will not adopt young chicks. The data at hand are 

 not conclusive beyontl this point. Adults whose eggs are pipped will sometimes 

 refuse to accept young birds in exchange for them, but usually during the last 

 week of incubation such changes may be made without difficulty. 



*E.'iperimental work with the colony was made almost impossible dm^ing the last 10 days of 

 June, at a time when the young birds were in the most interesting stage of dovelopraeut. High 

 ■winds and heavy rains, with a considerable lowering of the temperature, made it advisable to 

 disturb the colony as little as possible, as the birds frightened up from the ground were instantly 

 blown far out to sea and at times could not regain the island for hours. 



tSee page 4.'? of this volume. 



