NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 



65 



direction most free from other nests, and the egg was placed in this artificial nest. The 

 birds were allowed to find the new nest and settle down quietly over the egg each time 

 before being frightened away again. Table 22 shows the time required by each bird to re- 

 adjust to the new situation. The birds were driven away at intervals of 5 or more 

 minute.9. 



Table 22. 



*Did not find egg in half an hour. fUndisturbed by change. 



In each case but one the birds were greatly confused by the change in the posi- 

 tion of the nest and all showed a tendency to settle down at the original nest 

 site. As it was clear that the nest itself was not the stimulus to the localiza- 

 tion of this area, an attempt was made to control the immediate surroundings 

 of the nest. The following experiments with objects near the nest give some 

 insight into the mechanism of nest recognition: 



Experiment 4. 



In making their nests the sooties avoid the beach, very rarely going beyond the fringe of 

 bushes which marks the highest tide level. A few nests were made, however, at some dis- 

 tance from the bushes upon the sloping sand above high-tide mark. One of these on the 

 western shore, well removed from any other nest, was obhterated and replaced by two artifi- 

 cial nests, one 6 inches to the east, the other 6 inches to the west of the original site. 

 The egg from the original nest, which was rather light in color, was placed in the western 

 nest and another, marked with dark blotches, in the eastern one. The sand sloped from 

 east to west at about 15 degrees. 



Two minutes after these adjustments were made the sooty returned and, after inspecting 

 both nests, chose the strange egg in the eastern one (V" 20" a. m.). Half a minute later she 

 got up, went over to the western nest, and covered it. Her mate came and examined the 

 eastern nest. The first bird, attracted by his movements, went back to this nest and 

 settled down there. Her mate then flew away. She soon became dissatisfied with the 

 eastern nest and made several trips back and forth between the two. The visual stimulus 

 of the unoccupied nest seemed to furnish a stronger impulse than the tactual stimulus of 

 the nest occupied. At T^ 2.5'" a. m. the bird stood for some time between the two nests, 

 looking first at one, then at the other, and finally chose the eastern one. She was driven 

 away, returned, and took the eastern nest. Half an hour later she was driven away again; 

 she returned, again took the eastern nest, and paid no further attention to the other. 



The two nests were then obhterated and replaced by two others at the same distance from 

 the original site, but now north and south of it. The bird returned and chose the southern 

 nest, paying no attention to the other. Control tests showed that in this case the position 

 of the observer did not influence the choice of the nest. 



One egg was then placed in the original position, the other in a new nest .3 inches to the 

 eastward of this. The bird returned and chose the western nest, but soon left it for the 



