shore of the island was not adequately explored during nocturnal work so 
it is possible that an even larger proportion of the population was to be 
found in clubs. 
A considerable proportion, perhaps 10 percent, appeared to consist of 
prebreeding birds. On the southwest corner of the island a ratio of 16 
adults standing in pairs to 180 breeding adults* was obtained ( 8.8 percent) 
but this figure may well be low since most efforts to obtain returns in 
this area expended in obtaining known breeding birds. 
Eggs from 77 nests were tested for stage of incubation by flotation 
(16 nests containing one egg, 6 l nests containing two eggs). In the one 
egg nests 11 ( 68.8 percent) contained fresh eggs, 2 ( 12.5 percent) contained 
very slightly incubated eggs, 2 ( 12*5 percent) contained moderately incu¬ 
bated eggs, and 1 ( 6 .3 percent) contained heavily incubated eggs. In the 
two egg nests, one nest contained two rotten eggs. If this clutch is tem¬ 
porarily discarded from the sample, then, of the remaining 60 two egg 
nests, 9 ( 15-0 percent) contained fresh eggs, 22 ( 36.7 percent) contained 
very slightly incubated eggs, 14 ( 23-3 percent) contained slightly incu¬ 
bated eggs, 13 (21.7 percent) contained moderately Incubated eggs, and 2 
(3*3 percent) contained heavily Incubated eggs. 
If the percentages of eggs known to be incubated in one and two egg 
clutches are applied to the nest count we can arrive at an estimate of the 
clutch size for the population. My calculations Indicate a ratio of 6.36 
incubated two egg clutches to 1 Incubated 1 egg clutch for a mean clutch 
size of 1 . 86 . 
Of the 288 returns 18 had been banded on other islands. Six had been 
banded on Baker; 5 were from Jarvis; 3 were from Phoenix; 1 each was 
from McKean and Malden; and 2 were from Enderbury. 
Table 2 compares population and nest count data from the four July 
POBSP visits to Howland. 
Table 2. Comparisons of July nest counts of Blue-faced Boobies on Howland 
Island in 1963 , 1964-, 
1966 , 
and 1968 . 
Dates of visit 
1963 
vo 
OV 
1 — 1 
19 66 
1968 
July 6-8 
July 22-25 
July 12-22 
July lb-18 
Estimated population 
1500 
1800 
24-00 
1300 
Breeding population 
915 
1100 
1265 
925 
# of nests counted 
457 
551 
632 
446 
$ of nests/eggs 
45$ 
27$ 
33$ 
64$ 
$ of nests/young 
55$ 
73$ ' 
' 67$ 
36$ : 
$ of breeding birds/l egg 
1 4$ 
8 $ 
11$ 
18$ 
°lo of breeding birds that had 
naked or small downy young-$ 
2 9 $ 
ro 
-u 
13 $ 
0 
of breeding birds that had 
large downy young or larger 
young 
W 
VO 
(Vi 
46$ 
54$ 
25 $ 
* The figure of 180 is based on 
the number of nests 
or depende: 
nt young 
count ea in this area. At night, In actuality, the number and proportion 
of breeding birds was decidedly less, since many nests had but one adult 
at the nest, or accompanying the young birds. 
