BIRD ROOKERIES OF T0RTUGAS BARTSCH. 479 



the nests nearest one's position will be cautiously approached and then occupied. 

 A certain small percentage of the birds will remain on the nest, no matter how 

 violent the disturbance. 



It appears * * * that the shift at the nest is roughly a diurnal one, but 

 that at times it may not occur except once in 48 hours. * * * Apparently 

 most of the shifts are made at night. I attempted on many occasions to deter- 

 mine the hour of shifting by leaving a lantern near the nest and making ob- 

 servations during the night, but the light could not be arranged so as not to 

 frighten the birds, and their reactions consequently were not natural. The 

 birds would refuse to cover their eggs if the light were made intense enough 

 to be of value to me. 



Watson tells us that observation on 16 marked nests proved that 

 the period of incubation for the sooty is 26 days. We continue to 

 quote from him : 



During the first three days after the appearance of the young, the sooty is 

 reluctant to leave the young and nest on disturbance. Later the adults fly 

 away at the slightest disturbance, much as they do during the " laying " sea- 

 son. 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 near-by 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. * * * 



The adult would circle over the area and give a call; it would be an- 

 swered and random movements would give place to direct. The bird would 

 steer immediately for the source of the call. By peculiar chuckling sounds, 

 which are emitted at this period when mates return, one can be sure that the 

 proper nest has been located. I observed this many times during one evening. 

 After the young were 20 to 30 days old I have heard the young birds answer the 

 call of the parent back and forth a dozen times before the latter actually 

 alighted. 



Neither young nor old is quiet during this period of the nesting season. On 

 the contrary, the noise is practically doubled. In addition to the ordinary 

 sounds made by the adults and the new cries which are added at this time, there 

 is present the high-pitched, insistent " peep-peep " of the young terns. Mo- 

 mentarily the sounds of the adults will cease and the cries of the little ones 

 remind one very strongly of a poultry yard on a tremendous scale. 



Though the parents feed the young at any hour of the day, feeding can be 

 most easily observed at dusk. It has already been mentioned that the sooties 

 hurry home at nightfall in great numbers. From 4 until 8 p. m. this feeding 

 process keeps the island in commotion. The feeding of the young birds has 

 many interested spectators. While I have never seen the terns from the neigh- 

 boring nests, which may be observing the process, attempt to rob the young bird, I 

 judge from the actions of the feeding parent that such is occasionally the case. 

 If the parent happens to disgorge more than the young tern can take into its 



