NESTING ACTIVITIES OF NODDY AND SOOTY TERNS. 



63 



near the center and western end of the key. These grow to a height of 5 feet 

 and make many irregularities in the profile of the key which are probably 

 important as landmarks for the birds. Where the bay cedars have been 

 washed out the sand is covered by low fleshy herbs resembling the common 

 tumble-weed. At the beginning of the nesting season large numbers of these 

 weeds were torn up and piled along the beach to give the sooties nesting space 

 in the center of the island. 



At the western end of the island are some buildings, the warden's house and 

 the old detention hospital, which are the most conspicuous objects upon the 

 island. The group of buildings is about 50 feet in length and has a maximum 

 height of 15 feet. Three cocoanut palms (marked .x in figure 2) grow near the 

 buildings. The buildings, the jjalms, the clumps of bay cedars, and the piles 

 of brush along the beach present extremely complex visual stimuli to the birds 

 coming home from sea. Even at the most isolated nests the environment 

 presents a countless number of possible visual stimuli and it is never practica- 

 ble to control more than a very few of the stimuli which may be potent factors 

 in the birds' orientation. Long-continued studj^ of a single nest is prevented 

 by the fact that the birds soon become hopelessly confused by the changing 

 conditions of the ex]3ei'imcnt and cease to react definitely to the nest or nest 

 environment. The situation at every nest presents a distinct problem and in 

 no case has it been possible to reconstruct the entire process of orientation 

 from the data obtained at a single nest. The relation between particular 

 stimuli and the orientation of different birds has been determined, and a com- 

 bination of the results of observations at many nests gives a fairly complete 

 picture of the mechanism of orientation in the immediate neighborhood of the 

 keys. The experunents recorded in the following pages are typical of a large 

 number carried out at other nests and will serve to illustrate the chief phases in 

 the recognition of the nest and nest site. 



EXPERIMENTS WITH SOOTIES. 



RECOGNITION OF THE NEST SITE. 



The sooties become accustomed to the experimenter's presence very quickly 

 and often become so tame as to interfere seriously with the experiments, refus- 

 ing to be driven more than a few feet from the nest. After they have been 

 disturl)ed a few times their return to the nest is very prompt. The accom- 

 panying table gives the time elapsing from the moment when the brooding 

 sooty was driven from the nest until it had again covered 

 the egg. The time given is based upon the average of 6 

 birds during 5 successive trials at intervals of 5 minutes. 

 The average time which a tame sooty requires to rise from 

 the nest, fly 15 or 20 yards and return, is 12 seconds. This 

 is not appreciably changed by the substitution of a strange 

 egg for the one in the nest, or by the filling in or remodel- 

 ing of the nest by the experimenter, so long as the egg re- 

 mains in nearly its original position. A very slight change 

 in the lateral position of the egg, however, (causes a very 

 great increase in the time required by the bird to return to the nest and begin 

 to Ijrood the egg. In adjusting to such changes the .sooties show a very accu- 



