2i6 Papers from the Marine Biological Laboratory at Tortugas. 



appearance of the young as being strange to the adult sooty. The advent of 

 voung is sudden ; their movements, marking, coloration, etc., are striking 

 visual stimuli to the adult — especially to those which have no young of their 

 own and which are still brooding the egg. As previously stated, after dis- 

 turbance the adult returns to the nest first, then the young begin to struggle 

 back from the bushes ; and those shamming death in the open become active 

 and crawl back into the nest (these latter are especially liable to be de- 

 stroyed). In doing so, the young must often pass by areas defended by 

 brooding adults. Each adult begins to attack the small bird for exactl}' the 

 same reasons that it would attack a persistent sand-crab, or a noddy or 

 another sooty. The young bird by its peculiar cries and movements con- 

 tinues to offer stimulation to the adults, and not until the former reaches 

 its own nest or takes refuge in a motionless attitude and in a partially pro- 

 tected place do the attacks of the adults cease. As the young birds gain 

 in agility (and in experience) they rapidly learn to avoid defended areas 

 and to dart quickly by and under attacking adults. On many occasions I 

 have seen from 4 to 8 adults thus attacking a young tern, chasing it for 

 10 to 15 feet. 



At first the parent does not individually recognize its young, but reacts 

 to it by reason of its presence at the nest.^ Very early, however, there 

 is mutual recognition between parents and young. The parent will advance 

 toward its own young, even when several feet from the nest, and feed it, 

 often forcing it back to the nest by a cumbersome rolling process. The 

 young approaches the parent and nestles under its body. The parent ad- 

 vances toward the nest, often upsetting the young bird, but at the same time 

 advancing it. Both visual and auditory data are used in the mutual recog- 

 nition of parent and young. That auditory stimuli are functional can not be 

 doubted. Often I have observed a returning bird give a call a long way 

 ofif ; both the adult and the young answer the call and show changes in their 

 reactions. The parent at the nest will get up from the young and the young 

 will stand up, flap its wings, and leap several times a few inches in the air. 

 A moment later the returning bird will alight and feed the young. 



In order to observe the influences of auditory data on nest localization, I 

 watched the areas containing nests many times at night. One evening I 



' This was determined by exchanging young birds. The exchange is not noticed 

 when the birds are very young, but is noticed after the young is a few days old. My 

 notes are very inaccurate at this point. In one case I took the young at birth from 

 one nest (marked) to rear by hand. Two days later I put a new-born sooty in 

 this nest. The reactions of the adults were very curious. The male first came up 

 to the stranger young and gently struck it: the female came up and the young bird 

 attempted to nestle under her and to feed. Other birds from neighboring nests came 

 up, but were driven off. After this the young was treated in the usual way. There 

 is room for interesting and careful work upon this subject. (Some of the noddy 

 young are born with black juvenile phimage, some with plumage that is almost pure 

 white (see fig. 14, plate 4). Now, if a white bird is exchanged for a black one, or 

 vice versa, no disturbance is noticed on the part of the parent, even if there is a 

 difference of 3 or 4 days in the ages of the respective young birds.) 



