The Behavior of Noddy and Sooty Terns. 205 



In other cases botli and R will stay on the nest together for as long as 

 20 minutes. All the time R is " crowding " O more and more ; suddenly, as 

 R receives contact stimulation from the egg, a more pronounced movement 

 is made, thus forcing O aside. O is thus freed from contact stimulation. 

 gets up. nods to 7?. and flies away. R (now 0) turns the egg with the 

 beak and settles gradually down into the nest. 



A few examples are here quoted from my notes : 



May 12, 1907. One " relief " noted. O had been on egg for a long 

 time. R returned and gradually pushed aside. Nodding reaction very 

 much abridged. O circled around nest, then flew away. 



]\lay 13, 1907. had been on nest 3.5 hours. At 12 o'clock R appeared. 

 7? alighted upon the rim of nest and gave one or two upward flirts with the 

 beak. O was then gradually shoved aside. After had been pushed from 

 the &gg, both birds occupied the nest together for 15 minutes. then 

 disappeared. In a nearby nest the R perched motionless for 25 minutes 

 on the rim of the nest before attempting to push the O aside. 



The &gg is generally covered day and night. Occasional trips are made 

 by O to the water for drinking and for wetting the breast feathers. This 

 latter reaction has its value possibly in keeping the egg at the proper tem- 

 perature. The sun is so hot that if the tgg were left uncovered for any great 

 length of time it probably would not incubate. Occasionally, however, 

 will perch for 10 to 15 minutes on a nearby limb, leaving the tgg exposed. 



After I had observed some 12 to 15 cases of the above interchange of 

 activities at the nest, and found that the average time required for a shift 

 to take place was 2 hours, it occurred to me that the birds might have some 

 mechanism which might function as a time-sense (organic stimulation of 

 some kind, probably). In order to carry out the work carefully, one bird 

 (sex not determined) at each of 3 nests was marked with oil paint (aniline 

 dyes do not resist the action of the salt water). The nests were visited every 

 half hour, beginning at 6 a. m. and continuing until 8 p. m. The observa- 

 tions were continued 2.5 days. In table i, M refers to the presence of the 

 marked bird at the nest, U of the unmarked bird. The letter p shows the 

 presence of both birds at the nest. 



From the table it appears at a glance that the period of occupancy of the 

 nest is a variable one, ranging from 30 minutes to 5 hours. In some cases, 

 however, the sequence is somewhat regular (see May 22, 1907, nest No. 3, 

 from 8 a. m. to 8 p. m). It must be remembered that R is the bird which 

 controls the length of time occupies the nest. remains until R returns.^ 



It would be interesting to determine the stimulus which leads R to re- 

 turn to the nest. R does not always appear at the nest the moment the island 



' Experiments on distant orientation where one bird is removed from the nest 

 for several days at a time show that O will remain on the nest without going for food 

 for 24 to 48 hours, but not longer. After this period elapses. O will go for food, 

 leaving the nest and egg unguarded. 



