The Behavior of Noddy and Sooty Tciiis. 215 



From the reports of most of the residents, I found that the birds generally 

 leave from toward the middle of August to the first of September. That 

 they leave at night is most improbable, unless " at night " means early morn- 

 ing, at or shortly after daybreak. 



ACTIVITIES BY THE SOOTY AFTER THE EGG IS HATCHED. 



The period of incubation of the sooty egg is 26 days. This observation 

 is based upon 16 marked nests. The development of the young sooty is 

 deferred to a later place in this paper. 



The appearance of the young produces a profound change in the in- 

 stinctive reactions of the sooty. A general change in the disposition of 

 the bird is also noticeable. During the first three days after the appearance 

 of tjie young, the sooty is reluctant to leave the young and nest on disturb- 

 ance. Later, the adults fly away at the slightest disturbance, much as 

 they do during the " laying " season. It is interesting to observe at every 

 disturbance of a nesting-place how quickly the ground will be deserted 

 by both young and old, after the young have reached the age of 3 days. 

 As they leave, the alarm cry is sounded and the commotion spreads to all 

 the nearby nests. When quiet is restored the birds again alight near the 

 nest and gradually approach it. The young birds meantime have run to the 

 bushes, where they remain motionless after sticking their heads into the crotch 

 of some bush or depressing the body against any convenient solid object. 

 The protective coloring of the young sooties is marked. When motionless, 

 as above suggested, they are difficult to find. When the adult returns to 

 the nest, the young birds gradually come from their hiding-places at the 

 peculiar, clucking call of the parent. The parents (after the first few days) 

 recognize their own offspring with ease and accuracy, often going to meet 

 them as they emerge from the bushes. If by chance the wrong young bird 

 is met, it is struck with great force. Naturally this is productive of fights 

 between the adults. 



If disturbance occurs before the young birds are 3 days old, they will 

 " sham death " in the nest or advance a few feet from the nest and sham 

 death on the open sand. Lying outstretched upon the ventral surface of the 

 body, with head flat upon the ground, it is with difficulty that one believes that 

 life is present. The slightest contact stimulation will cause the young bird 

 to attempt to get on its feet and struggle away. 



From I to 6 days is a critical period in the life of the young sooty. Hun- 

 dreds of them are killed by the adults, and were it not for the "death-sham- 

 ming instinct " thousands would perish. This tendency on the part of the 

 adult birds, especially of gulls, to kill the young has often been commented 

 upon by nature students, but the phenomenon has not been very well un- 

 derstood. The situation as it exists at the Tortugas colony is, I believe, 

 not difficult of understanding. In the first place, we must consider the 



