NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 77 



(6) I replaced a pipped sooty egg with a young noddy. The sootiea attacked the young 

 noddy and drove it from the nest. A third sooty from another nest came and helped in the 

 attack and I was forced to interfere to save the chick's life. 



Such reactions are by no means invariable. A sooty will sometimes refuse 

 to accept a sooty chick in exchange for its egg or again will fail for some min- 

 utes to notice the presence of a young noddy in its nest; but in general the 

 different appearance of the chicks of the two species calls out different instinc- 

 tive behavior in the adult sooties. 



The greatest variation in the behavior of the adult sooties occurs in their 

 reaction to strange chicks of their own species. Ordinarily they do not dis- 

 tinguish between their own young and other sooty chicks of about the same 

 age during the first few days after hatching. 



Experiment 1.5. 



I replaced a 3-day chick in a nest with another of the same age. The parent returned to 

 the ground near the nest, rushed forward, and threw out the nearest chick (her own), drove 

 him into the bushes, then walked back and covered the other. I placed the abandoned 

 chick in a nest with another of about the same age. One of the parents of this chick 

 returned and accepted both young. Three days later both chicks were still in the nest. 



Chicks from 1 to 4 days of age may almost always be interchanged without 

 inducing any marked change in the behavior of their parent.?. Many such 

 exchanges were made and almost without exception the changelings were 

 adopted without hesitation. Until the age of 4 days the sooty chicks are not 

 recognized individually by their parents. At about that age a change takes 

 place in the behavior of the young sooty. Before this time it feigns death 

 when attacked by adult birds or when removed from the nest to strange 

 surroundings. On the fourth or fifth day it becomes much more active, at- 

 tempts to crawl out of the nest and hide in the bushes at the experimenter's 

 approach, and attempts to run away when attacked in the open by older soot- 

 ies. After the age of 10 days it spends most of the daylight hours lying hidden 

 among the weeds and bushes near the nest and comes out only in the evening 

 to receive food from the parents. With its increasing activity it seems to 

 form a set of habits with respect to its nest very similar to those of the adult 

 birds. In its own nest it lies quiet when attacked by strange adults, which 

 happens frequently, for the sooty rarely misses an opportunity to take a sly 

 dig at her neighbor's chick; but outside its own nest the chick, after the fourth 

 day, scrambles away actively when attacked. This seems to furnish the first 

 criterion by which the adults recognize their own young. 



Experiment 16. 



(o) I interchanged a 1-day and a 6-day chick. The parents at both nests were much 

 disturbed, refused to cover their nests, and poked the young ones with their beaks. The 

 younger chick lay still in the nest and after a short time seemed to have been adopted. 

 The older chick started to run away and was immediately attacked savagely and driven 

 away among the bushes. 



(6) I exchanged a 3-day and a 5-day chick. The younger bird was accepted immediately 

 by the parents of the other. They did not seem to note the change. When the adults 

 returned to the other nest the 5-day chick ran away. They attacked it at once and drove 

 it away to a distance of 4 feet, where it received so much abuse that it gave up and refused 

 to run further, feigning death. I returned it to the nest. The adults examined it and 

 pushed it away roughly. This time it lay quite motionless, however, and the older birds, 

 after driving away a trespasser, adopted it. 



