WHITE TERN 



Spring 



Totals 



1,128 



Summer 



Fall 



1,656 



1,896 



Winter 



1,632 



Figure 29. — Sea bird abundance in subareas in the study area. The squares of the table repeat the 

 geographical layout of figure 28. The upper figure in each square is the density in birds per 10 

 linear miles (18.5 km.) of observation; the lower figure is the estimated population, weighted by 

 the effective range of observation of each species. Density and population figures are daily 

 averages during each season. — Continued. 



Direction of Movement 



The direction in which birds were flying was 

 noted at the time of each sighting. Often it was 

 not possible to assign one particular direction 

 of movenjent because many species, especially 

 some of the shearwaters and petrels, have a 

 meandering flight that is probably indicative of 

 random searching for food. Figure 30 shows 

 the percentage of birds of different species seen 

 flying in each direction during each season. 

 The data are most conclusive for species that 

 migrate directly through the study area, e.g.. 

 Sooty and Slender-billed Shearwaters. Direc- 

 tion of movement of albatrosses almost always 

 coincided with that of the ship, so they were 

 not included in the figure. Biases to the data 

 include alterations in course as a reaction to 

 the ship, and the tendency of birds breeding in 

 tha main Hawaiian group to radiate out from 

 land in the morning and to return to land in the 

 evening. Because the ship normally spent little 

 time near land, records of direction of move- 

 ment of species that breed in substantial num- 

 bers in the main Hawaiian group, e.g., light- 

 phase Wedge-tailed Shearwaters and Sooty 

 Terns, may be biased by the chance arrival of 

 the ship in that area in the morning or after- 

 noon. 



Certain trends are probably significant, how- 

 ever. Species that breed in the main Hawaiian 

 group generally tend to fly north (or northeast 

 or northwest) in spring and summer, and east 



to southwest in fall and winter. Direct migrants 

 head north or northwest in the spring in con- 

 vincing numbers, and south in the fall in equally 

 convincing numbers. Species that breed in the 

 South or North Pacific and winter in the study 

 area, e.g., Juan Fernandez Petrel and Leach's 

 Storm Petrel, fly north to west in the spring 

 and summer and south to east in the fall and 

 winter. 



Daily Cycles of Activity 



I totaled numbers of each of the more com- 

 mon species, according to the number of hours 

 between sunrise and sunset they were observed, 

 to indicate their activity cycles (fig. 31). For 

 some, e.g., Laysan Albatross, the small sam- 

 ple size and the large erratic fluctuations in 

 numbers indicate insufficient data. Others show 

 fairly smooth curves that comply with the gen- 

 eral impression of activity 1 have formed from 

 many hours of observation. Hours six and 

 seven after sunrise overlap hours seven and 

 six before sunset in figure 31 as a compro- 

 mise solution to the problem of including some 

 birds in both a.m. and p.m. totals on short 

 (11-hour) winter days, but not on long summer 

 days of 13 hours or more. 



Four basic patterns of activity are evident: 

 1. Peak activity near mealtimes aboard the 

 ship. The Black-footed Albatross showed 

 this pattern — which is not unexpected be- 

 cause this bird is a scavenger. 



78 



