NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 67 



The stake and egg were moved 2 feet to the westward (/) and another egg was placed 

 where they had just been (e). The bird returned to her alighting place, examined the egg 

 and stake (/), then covered the egg in the former position (e). She was driven off and re- 

 turned to her alighting place immediately. Another bird alighted near the unmarked nest 

 (e) and disputed her possession of it, so she went to the egg by the stake and covered it. 

 A moment later she got up and went back to the unmarked nest. The strange bird made 

 way for her and she covered the egg there. This is the position in which she is shown in 

 plate 7. It is in the direct line between the alighting place and the original site (a). The 

 egg and stake were next placed between this 

 position and the original nest site. The bird con- 

 tinued to occupy the unmarked nest. The egg 

 and stake were returned to their original position, 

 but the bird did not again take them. 



This series of observations offers several 

 interesting points : 



(1) In comparison with the sooties in 

 crowded localities, the bird under obser- 

 vation showed a remarkable plasticity. c 

 Attempts to duplicate the experiment in 



JJV-L-J r-ii i j. i FIG. 3. The path followed by the sooty 



crowded districts failed completely. in ex p er i m ent 5 when the egg (c) was 



(2) The constancy of the place at which moved away from the stake (a). The 



the bird alighted is very striking. How reaction to the 4 chief visual stimuli 



. , , of the region is shown. 



this place was recognized could not be 



determined, but it was near an open place in the bushes which offered many 

 more visual criteria than did the immediate surroundings of the nest. The 

 certainty with which the direction was taken from this position to the nest 

 is also unexplained. It was to a certain extent independent of the appear- 

 ance of the immediate vicinity of the nest. The question of orientation from 

 the alighting place will be taken up later. 



(3) The relative force of the egg and stake in determining behavior gives 

 some insight into the bird's habits with respect to the nest environment. 

 The stake had been planted 2 weeks before to mark the site of another nest, 

 at that time destroyed. In 2 weeks it came to occupy an important place in 

 the bird's recognition of her nest, perhaps taking the place of the obliterated 

 nest in her system of habits. That it was not the fundamental factor in locat- 

 ing the nest is shown by the fact that after a few failures to reach the nest by 

 going to the stake the bird paid little further attention to the latter. 



EXPERIMENT 6. 



A nest in a rather crowded area was chosen for this experiment. It was 8 inches north of 

 a stake which had been planted a week before to mark another nest, and near the center of 

 a triangle (figure 4), the northern side of which was formed by a line of very dense weeds 

 (c), the western by a very dense clump of weeds (/) and the stem of a palm leaf (d), the 

 southern side (a-g) being open. The stake shown in figure 4, at the right of the nest (6), 

 was originally at h. I drove the bird from the nest she circled above the island and came 

 back immediately, alighted 6 feet to the eastward of the nest, and walked almost straight 

 to it (along the line a-b in the figure). This was repeated twice, her path in returning 

 being each time the same. 



The stake at h was by far the most conspicuous object near the nest, and to test its effect 

 upon the bird's reaction I moved it from its position south of the nest to an equal distance 

 north of the nest. I also constructed a new nest, north of the stake at the position marked c. 

 The relation between this nest and the stake was now the same as the relation of the 

 nest at b and the stake had been before the change. The bird's own egg was placed in the 

 nest (c) and a strange egg was placed in b. The bird returned by the path from a and took 



