3o8 Papers from the Marine Biological Laboratory at Tortugas. 



extent) congregated in a zone near the blunt end, and lesser pale grayish vinaceous 

 and deep burnt sienna spots sparsely scattered over the rest of the egg. 



The general disposition of the sooty, hke that of the nodd}', changes 

 after the egg is laid and in the same way. Some of them become far bolder 

 than the noddies in a corresponding situation. It was possible for me to 

 lie down within a few inches of a brooding sooty and have it remain on 

 the nest indefinitely. If the hand is extended toward the sooty it will attack 

 vigorously, but I have never had a group of flying sooties attack me as I 

 approached the vicinity of their nests, as was sometimes the case when I ven- 

 tured too near the nests of the noddies. The birds- are very variable in 

 this respect. When one approaches a neighborhood containing many nests, 

 the majority of the birds will fly up into the air, circling round and round, 

 screaming all the while. If one remains quiet, the birds will gradually re- 

 turn and cover the eggs. Gradually the nests nearest one's position will be 

 cautiously approached and then occupied. A certain small percentage of the 

 birds will remain on the nest, no matter how violent the disturbance. 



My study of the instinctive reactions of the brooding sooties was again 

 beset with difficulties because of the large number of birds present. My 

 method of studying their behavior was similar to the one just described for 

 the noddies. I would choose a favorable spot where several nests containing 

 marked birds could easily be observed at once, then record what went on at 

 each separate nest. I speedily found that the nesting reactions of the brood- 

 ing sooties are quite different from those of the noddy. I would sometimes 

 spend 4 to 6 hours at the nests without seeing a single bird leave its nest 

 except to make short excursions for the purpose of fighting or to obtain 

 water. The following day would sometimes find the same bird on the nest, 

 sometimes its mate. In order more accurately to obtain data upon the ques- 

 tion of the division of labor between the two sexes, I made a table (table 2) 

 similar to the one presented for the noddies. At first the observations were 

 taken every half hour, but as soon as it became apparent that so many ob- 

 servations were needless the period was lengthened. 



It appears conclusively from table 2 that the shift at the nest is roughly 

 a diurnal one, but that at times it may not occur except once in 48 hours. 

 All of the factors in this reaction do not appear in the table. Apparently 

 most of the shifts are made at night. I attempted on many occasions to 

 determine the hour of shifting by leaving a lantern near the nest and mak- 

 ing observations during the night, but the light could not be arranged so as 

 not to frighten the birds, and their reactions consequently were not natural. 

 The birds would refuse to cover their eggs if the light were made intense 

 enough to be of value to me. 



Then again, in certain cases, the shift is not made within the 24-hour 

 period. Where is the absent bird? All my efforts to discover its presence 

 near the nest failed. Does it go such a great distance for food that it must 



