38- 

 3G' 

 34- 

 32- 

 30- 

 28- 

 26- 

 24 

 o 22- 

 2 20- 



i '^ 



16- 

 14 



i: 



10 

 8 



G 



1^ Terns 



I I Pelreli 



] nn]geUlledS^earwMer 



□ 0"»" 



] Sooty and Slender billed Sheaiwaleis 



I Altutiouei 



MAR APR MAY lUNE JUL* SEPI OCT NOV DfC IAN FEB MAR «PR MAY JUNE 



Figure 25. — Monthly abundance of species groups 

 in birds per hour of observation. 



June 1965 



Although total density decreased considerably 

 from May 1965, it still was higher than in June 

 1964. Sooty Terns and Wedge-tailed Shear- 

 waters accounted for 64.6 percent of the total 

 population. Most of these species were in the 

 high-density center around Oahu, although num- 

 bers of dark-phase Wedge-tailed Shearwaters 

 were seen at the southeastern corner. Numbers 

 of Juan Fernandez Petrels increased consider- 

 ably over May 1965 and were far ahead of their 

 total in June 1964. Black-winged Petrels re- 

 mained constant. No migrants were reported. 



Table 4 and figure 25 show the monthly abun- 

 dance of species or species groups in the study 

 area in terms of birds per hour of observation. 



COMPONENTS OF ABUNDANCE 

 AND DISTRIBUTION 



The abundance and distribution of sea birds 

 at sea are governed by the interplay of an 

 unknown number of environmental factors. In- 

 vestigation of the relative abundance and dis- 

 tribution of species under several environmental 

 conditions at one time is beyond the scope of the 

 present work; however, it may prove fruitful to 

 document the abundance and distribution of each 

 species relative to a single environmental 

 variable. Such treatment may suggest mean- 

 ingful relations involving several variables that 

 can be subjected to more analytical scrutiny at 

 a later date. I have chosen to examine the 

 following categories: the islands of origin of 

 sea birds in the study area and the ways in 

 which sea birds utilize the area; the abundance 

 of various species relative to distance from 

 nearest land; the abundance of various species 

 in different portions of the study area; the 

 direction of flight of various species; the daily 

 cycles of activity; the abundance of species 

 relative to five climatic parameters; and the 

 participation of various species in flocks. 



Islands of Origin of Sea Birds 

 Recorded in tlie Study Area and Modes 

 of Utilization of the Area 



The avifauna of the study area is a composite 

 of birds that breed throughout the Pacific Ocean. 

 Several species breed in only one known locality 

 and leave no doubt about their provenance; 

 however, most have populations that breed on 

 more than one island group. To establish a 

 probable point of origin, factors such as breed- 

 ing phenology, behavior in the study area, and 

 extent of orientation to land must be considered. 

 At least two populations from different islands 

 of origin of the two most abundajnt species. 

 Sooty Tern and Wedge-tailed Shearwater, are 

 present at certain times of the year. 



It is useful to distinguish between two basic 

 patterns of utilization of the study area by the 

 species that breed north or south of the central 

 Pacific--direct migration and "wintering." 

 Direct migrants seldom feed in the study area 

 but pass through rapidly as they travel between 

 temperate climatic zones. Wintering birds re- 

 main in the study area during all or part of 

 their nonbreeding season and depend upon the 



62 



