82 HOMING AND RELATED ACTIVITIES OF BIRDS. 



The period of incubation may be lengthened also to some extent, but this 

 differs in different birds and its limit was not determined. 



In 10 nests younger eggs were substituted for pipped ones. After 2 days 

 one of the nests was abandoned. During the following 5 days 6 of the eggs 

 hatched. The remaining birds were still sitting at the end of 7 days, when 

 observations had to be discontinued. 



A dead egg was substituted for a pipped one, somewhat earlier in the season. 

 Upon the following day it was found that the parents had adopted a 3-day old 

 chick and were brooding both it and the egg. 



This observation and the abandonment of the single nest in the other 



experiment suggest that the length of the incubation period is conditioned by 



an internal rhythm, which does not determine, however, an exact period of 



26 days, but permits a range of variation of 2 weeks or more around this 



period. 



INTEGRATION OF COMPLEX HABITS. 



Two of the most striking features in the colonial life and behavior of the 

 terns are the enormous number of habits which they exhibit and the discon- 

 nected, impulsive character of their activities. The number of habits involved 

 in the process of orientation has been illustrated in the earlier experiments. 

 By the time that the experimental work had been cari'ied far enough to give 

 an insight into the mechanism of orientation the eggs were all laid and the 

 birds had been reacting to the situation at the nest for some weeks. Hence 

 there was little opportunity for study of the formation of the habit complexes. 

 In general, as might be expected in a colonial bird, the most definite and firmly 

 established reactions are associated with other birds or their nests. The chief 

 landmarks used by many sooties for the final location of their nests are the 

 nests of their neighbors, and it is apparent that the presence of these neighbors, 

 defending their nest areas, tends to give a greater stimulating value to the nests. 

 The birds whose nests were in very crowded areas showed a much lower plas- 

 ticity in adapting to changed conditions than those whose nests were somewhat 

 isolated, hesitating to occupy nests only a few inches from their own nest sites 

 and offering little defense of such nests against trespassers. The behavior of 

 the birds suggests that in the early part of the nesting season exchanges of 

 nests must be of relatively frequent occurrence. An interesting example of 

 the diffidence of the birds in appropriating new nests is given in the following: 



Two sooty nests, 18 inches apart, were obliterated and both eggs were placed in a new 

 nest half way between them. The parents of both eggs returned to their nest sites, missed 

 the eggs, and began to look about for them. One caught sight of the eggs in the new nest 

 and covered them. The other at once drove her off and took the ne.st. The first, after 

 standing for a few moments at her nest, returned to the new nest and drove off the second. 

 This was repeated several times, the bird occupying the nest offering little resistance to the 

 attacks of the other but returning fiercely to the attaclc after a visit to her own empty nest. 

 Finally one bird rolled an egg back to its nest and both settled down contentedly. 



In small matters, such as changes of a few inches in the position of the nest, 

 changes in the appearance of the egg, young, or nest, the habits of the birds 

 may be quite readily motlified, but this seems to hold true only within certain 

 limits. Many attempts were made to shift nests to the edge of the beach 

 with the puipose of transferring them to rafts in the hope of ultimately trans- 

 planting a part of the colony to other keys, but however slowl}' such shifts 



