STERNA FUSCATA 



DECEMBER 1%4 



JANUARY 1965 





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Figure 22c. — Distribution of Sooty Tern, December 1964 to March 1965. 



Sooty Terns were present in large numbers 

 every month of the year (figs. 22a-d). The 

 pattern of distribution is somewhat clarified if 

 the birds are divided into two groups, one with 

 a southerly distribution and a density center 

 usually at the southeast corner, and the other 

 with a northerly distribution and a density cen- 

 ter within 50 miles of the Hawaiian Islands. If 

 the division is made along lat. 16° N., which 

 roughly corresponds to the line of least density 

 between the two density centers, figure 23 ex- 

 presses monthly abundance of the two groups. 

 They may be separate populations or different 

 age groups of the same populations. The peaks 

 and declines in abundance of the two groups 

 were clearly out of phase with each other. 



The abundance pattern of the northern popu- 

 lation appeared to be directly related to the 

 breeding cycle of the Sooty Terns on islets off 

 Oahu. In general, the northern half of the 



study area had large numbers when breeding 

 was underway. At the conclusion of the breed- 

 ing cycle the Sooty Terns left the islets and the 

 pelagic area around them. Numbers remained 

 low from September through December but in- 

 creased again in January and remained high 

 until June, when the study ended. 



The abundance changes in the southern half 

 of the study area were less clear-cut, perhaps 

 because the birds that constitute a large part of 

 the population may come from the Line Islands, 

 where breeding cycles are on a 6-month sched- 

 ule (Ashmole, 1963). A peak occurred in May 

 1964, followed 5 months later by one in October. 

 A subsequent decline was followed by two lesser 

 peaks in January and April 1965. In May 1965 

 numbers in the southern half were at a record 

 low, in contrast to May 1964. Although the data 

 indicate some cycle other than an annual one, it 

 is not possible to ascribe a 6-month cycle to 



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