80 HOMING AND RELATED ACTIVITIES OP BIRDS. 



INSTINCT AND HABIT IN THE NESTING ACTIVITIES. 

 MODIFIABILITY OF INSTINCTIVE ACTIVITIES. 



The activitie.s of the birds centering around the nest during the breeding 

 season may be roughly divided into mating, selection of the nest site, construc- 

 tion of the nest, laying, incubation, and the rearing of the young. During 

 each of these periods a special group of instinctive acts is called into play and 

 the behavior of the birds is modified quite suddenly at the transition from 

 one period to the ne.xt, as in the sudden appearance of the constant brooding 

 activities after the egg is laid. Watson found that birds which are quite 

 timid become aggressive as soon as the egg is laid and either refuse to be 

 driven from the nest or attack intruders actively. He was able to induce an 

 early appearance of this change in behavior by placing an egg in a newly 

 completed nest, and thus raised the question of the relation of these changes 

 in instinctive behavior to external stimuli and to internal processes. 



I have obtained a small amount of evidence bearing upon this question, 

 but the data are fragmentary and a great deal more time may be sjjent profit- 

 ably upon this particular aspect of behavior. Owing to the lateness of the 

 season when the work was begun no observations were obtained upon mating 

 or the construction of the nest. 



Experiment 19. 



After some ditficulty a newly completed noddy nest was found. The building activities 

 of the owners had ahnost ceased, but no egg had been laid and the birds were not occupying 

 the nest regularly. At the first observation, one of the birds was perched on a dead limb 

 near the nest and the other was not in the neighborhood. A noddy egg was placed in the 

 nest. This was early in the morning. No change in the activity of the bird at the nest was 

 observed during the day. It disappeared occasionally, but did not show any reaction to the 

 egg. The mate was not seen. 



On the following morning a bird was found brooding the egg. Its mate was on the dead 

 limb near by. When I approached both birds defended the nest, the one on the perch flying 

 into the air and striking at me, the sitter refusing to leave the nest. I caught the latter and 

 marked it with a spot of paint. The marked bird was occupying the nest when it was 

 observed later in the day. 



On the third day only the unmarked bird was seen at the nest. It spent the day sitting 

 on the perch or on the edge of the nest, paying no attention to the egg. 



On the fourth day the marked bird was again observed at the nest. Both birds were 

 upon the nest, the marked one brooding the egg, the other standing on the edge of the nest. 

 Both flew up at my approach. The marked bird returned immediately and covered the egg. 

 The unmarked one returned, hovered about for a short time without alighting, and then 

 flew out to sea. 



On the following day the marked bird was observed on the perch. At this time the 

 unmarked bird was on the nest over the egg. For as long as I watched, this bird did not 

 assume the brooding position, but stood upright and turned about, moved to the edge of 

 the nest, and flew away rapidly when disturbed, thus showing the behavior of the layei 

 rather than of tlie sitter. 



The marked bird was on the nest for the greater part of the sixth day. The unmarked 

 bird was seen on the edge of the nest, but did not cover the egg. 



On the seventh day a second egg had been deposited and the unmarked bird spent the 

 day in brooding the eggs. After this time the birds seemed to take turns regularly in 

 brooding the eggs. 



In this case it seems clear that only one of the birds was stimulated to the 

 brooding phase of activity by the sight of the egg. The other retained almost 

 wholly the behavior of the layer up to the time when the second egg was depos- 



