

Figure 7. Movements of marked unemployed Laysan albatrosses on Sand 
Island. A total of 3,218 Laysan albatrosses, presumed to be un- 
employed, were marked, January through March 7, 1957. ‘Two were 
later found on nests, as noted. 
Plots were set up to determine if there is a daily rhythm in 
numbers of birds present on the ground and in the air. Three plots were 
marked off on runways for aerial counts and 6 in nesting areas for ground 
counts for this study. The purposes were to determine to what extent 
hazard to aircraft varied from hour to hour, day and night, and also to 
determine at what time of day populations might be most nearly static 
so population studies could be conducted during this optimum period. 
Hourly counts were made on the plots throughout the night of November 
20 and the day of November 23 (a 24~hour period). The night counts 
showed a maximum population on the ground, with very little hourly 
variation. Figure 8 shows the results of the ground counts during the 
daylight hours from 7 asm. to 6 p.m. A rapid decline occurs at dawn, 
with a minimum at 8 a.m., followed by a gradual rise until the maximum 
is reached again the following night. Figure 9 shows the curve for 
birds in the air over the runways to be the reverse of those on the 
ground, with a maximum peak at 8 a.m. This definitely suggests an 
inverse relationship between the two groups during this period in the 
early part of: the breeding cycle. It should be noted that the ground 
counts showed a greater fluctuation and an earlier return to maximum 
populations in the afternoon on the part of the black-footed albatross 
which was already advanced in egg laying while the Laysan albatross had 
hardly begun to lay. The population of Laysan albatrosses on the ground 
plots did not vary more than 4.5 percent between 8 and 12 a.m. Similar 
counts of all birds on the ground were made in an area around the 
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