19S Papers from the Marine Biological Laboratory at Tortugas. 



upon the nature of the surface. It is rarely over 5 cm. in depth, even in 

 loose sand. 



The northern and northeastern sections of the island are free from bushes, 

 but are covered by a shallow growth of Bermuda grass. These areas con- 

 tain by far the largest number of nests. The group photographs of the 

 sooties appended (see especially figure 17, plate 6) are taken at one or 

 the other of these sections. The eggs in these areas are laid literally on the 

 grass and bare earth in no kind of nest structure. The eggs are often de- 

 posited in open sandy places, but nest depressions are not always made, even 

 where the nature of the surface easily permits it. A reference to figure 18, 

 plate 6, will show quite clearly the absence of any complex nest-structure. 



A rather interesting variation in nest-structure appears among certain 

 nests which are built under the bay-cedar bushes. The leaves from the 

 bushes sometimes form a carpet over the sand. The nesting sooties often 

 gather up these leaves and place them around the rim of the depression. 

 Under no circumstances are the leaves collected from a distance further 

 than the birds can reach with their beaks while remaining in a sitting posture 

 in the nest. 



The nests of the sooties are assembled into groups. Roughly speaking, 

 there is a southeastern, a central, a northern, and a northeastern group. 



An approximate count of the total number of the sooty nests was made 

 in the following way : Those parts of the surface of the island containing 

 nests were subdivided into ten separate areas. The number of square feet in 

 each area was next determined. The average number of nests (spots where 

 eggs were deposited) per square foot was then determined separately for 

 each area. By means of these data, the total (approximate) number of nests 

 on the island was found to be 9,429. Multiplying by two, as in the previous 

 case, we have 18,858 as the total number of adult sooties. It may be said 

 that the above determination was made late in the brooding season, after all 

 the eggs had been laid. It may also be of interest to note that in localities 

 where the nests are very numerous they often are not more than 10 to 12 

 inches apart. On account of this close grouping of the nests, and of the 

 quarrelsome nature of the brooding birds, exact localisation of nest and 

 recognition of nest and mate easily became the most important features in 

 the lives of the sooty terns. This situation affords a convenient starting- 

 point for a psychological study of the behavior of these birds. My tentative 

 beginnings in this field are described on pages 221 ff. 



REACTIONS OF THE NODDIES OBSERVED IN NEST-BUILDING. 



My notes, written during the observations, contain a large amount of 

 material relating to the way in which the noddy and sooty nests are con- 

 structed. The greatest difficulty in obtaining accurate notes lies in the fact 

 that in neither of these two species are the differences in visual appearance 



