NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 69 



EXPERIMENT 7. 



A nest near that described in experiment 5 was placed in a pan of sand, 1 foot square and 

 2 inches deep, so that it was raised 2 inches above its former level. The owner of the nest 

 always alighted 6 feet to the eastward and walked by a somewhat zigzag path to the nest. 

 When the pan of sand was first put in place she approached, walked around it, and 

 inspected it carefully. She next stopped, peered about in all directions, then stepped up 

 to the higher level. She did not cover the egg, but walked about in the pan and finally 

 backed over the edge and fell out. She spent some time standing near the pan, then stepped 

 up on it again, and after 8 minutes covered the egg. Half an hour later she was driven away. 

 She returned at once, spent 2 minutes in preening her feathers, then stepped up into the pan, 

 and covered the egg immediately. 



The pan was next raised to a height of 6 inches from the ground. The bird returned to 

 her alighting place, walked toward the pan, hesitated, walked around it several times, but 

 made no attempt in half an hour to get upon it. With the pan in this position she could not 

 see the egg from the ground. The pan was then lowered to a height of 4 inches. The bird 

 came from her alighting place to the pan, climbed into it with difficulty, putting one foot 

 in it and fluttering up, and covered the egg in 2 minutes. The nest was left in this position 

 until the following day, when the experiment was resumed. The bird was then driven off 

 several times and each time returned to her usual alighting place, walked to the edge of the 

 pan, then fluttered up to the egg. The nest was raised to 5 inches and was taken immedi- 

 ately. It was next raised a foot from the ground. The bird walked around it for 5 minutes 

 before flying up and covering the egg. It was left in this position for an hour and was then 

 raised to a height of 2 feet. The bird returned to her alighting place, walked to the foot 

 of the post supporting the nest, and seemed very much puzzled by the new condition. She 

 walked round and round the post, looking up at the nest, but in 10 minutes made no at- 

 tempt to fly up to the pan. The pan was lowered to 6 inches and was occupied immediately. 



The pan and its support were moved 3 feet to the westward and fixed at the same height. 

 The bird came to the former nest site and wandered about for 10 minutes, but did not seem 

 to see the nest in the new position. The nest was next moved 6 feet to the eastward of its 

 original position and fixed at the bird's alighting place. The bird came to the ground beside 

 the nest and fluttered up and covered the egg immediately without a sign of dissatisfaction. 

 The pan was returned to its first position and fixed at a height of 6 inches. It was accepted 

 at once. 



Two days later the same bird was upon the nest. The egg had hatched in the meantime. 

 The pan was raised to the top of a post 2 feet above the ground. The bird walked around 

 the post for half an hour without seeming to look up at all. She then flew away and was 

 not seen for the remainder of the day. The next day she was again in the nest. When 

 driven off she returned and alighted directly upon the nest. 



The pan was raised to a height of 5 feet. The bird came to the ground and walked 

 around under the pan for several minutes, looking up. Then came the most interesting 

 part of her entire series of reactions. She flew up to a height of 2 feet and went through the 

 motions of alighting, just as though the pan were still at this height, then dropped back to the 

 ground. She repeated this five times in exactly the same way, then walked about for a few 

 minutes, looking up, and finally flew up to the pan and covered the nestling. She was 

 frightened away again, returned to the ground, and flew up to the pan immediately. 



This experiment brings out three facts quite clearly. First, adjustment to 

 changes in the height of the nest is not made any more readily than to changes 

 in the horizontal position. Second, shifting the nest along the pathway toward 

 the alighting place disturbs the birds less than a change in the position of the 

 nest in any other direction. Third, there is strong tendency to replace visual 

 motor reactions by habits based upon kinsesthesia, resulting in reactions like 

 those shown by the rat in the adjustable maze. 



Clearly, in all these experiments the birds were influenced to some extent in 

 the recognition of their nests by visual objects in the immediate neighborhood. 

 Such objects were other nests, debris which had been in place since the nest 

 was made, and objects recently added to the situation. It appears that long 



