Sea Bird Food and Ocean Surface Fauna — Ashmole and Ashmole 
7 
is not often available in large numbers. However, 
either the Red-footed Booby, Sula sula (Lin- 
naeus), or the Masked (or Blue-faced, or 
White) Booby, Sula dactylatra Lesson, might 
be useful. 
S. sula nests in colonies in bushes or trees, 
and regurgitates readily, at least when with 
young. The birds roost on many islands through- 
out the year; on Christmas Island, where the 
species nests on the main island, there is some 
breeding at all times of year. S. sula catches 
some of its prey below the surface by diving 
from a height, but has also been recorded catch- 
ing flying fish in the air; it is thought some- 
times to fish at night (Murphy, 1936). S. sula 
probably fishes mainly within a hundred miles 
of its home island (Murphy, 1936; Royce and 
Otsu, 1955, who almost certainly mistook 
5’. sula for S. dactylatra ). In a small series of 
samples from Oahu, Hawaii (Ashmole and 
Ashmole, in press) we found an average of nine 
items per sample, of which 64% were fish, 
36% squid. Nearly 75% of the fish were 
identified, so that about 400 identifiable fish 
could be expected per 100 samples from this 
species. The fish found most frequently were 
Exocoetidae and Gempylidae, while all the 
identified squid were Ommastrephidae. Half 
of the fish were between 8 and 12 cm long, 
while there were appreciable numbers in the 
12-16 and 16-20 cm groups; however, only 
a few squid with mantle length of more than 8 
cm were present in our samples. The main 
advantages of this species are that it is available 
in some places throughout the year, and takes 
fish (and perhaps also squid) larger than any 
taken by the terns previously discussed. 
We have had little experience with S. dactyl- 
atra, but relevant data have been presented by 
Dorward (1962). It would have advantages 
similar to S. sula, but probably ranges even 
farther from its breeding colonies. 
DISCUSSION 
It is clear that the use of analysis of the food 
of each of these bird species has certain merits, 
but also certain disadvantages. We suggest that 
valuable data could be obtained by making use 
of a group of bird species selected for the 
particular problem under investigation. For in- 
stance, to investigate the effects of seasons in 
the ocean around Christmas Island, it would 
probably be best to obtain a series of regurgita- 
tions, perhaps twice each month, from the three 
species Anous tenuirostris, A. stolidus, and S. 
sula. These samples would include animals 
ranging in size from less than 1 cm to at least 
20 cm. The regurgitations from A. tenuirostris 
probably would represent sampling largely 
within 5 miles of the island, those from A. stol- 
idus sampling largely within 50 miles, and those 
from S. sula might have been obtained as much 
as 100 miles away. On the other hand, for a 
long-term study concerned primarily with the 
detection of differences in the fauna of an 
oceanic area at the same season in a series of suc- 
cessive years, it would probably be most econom- 
ical to collect samples from Sterna fuscata, 
choosing those months when this species has 
young. In any study, collection of samples from 
Gygis alba would be useful in providing animals 
in excellent condition and wide variety, to form 
the basis of a reference collection of the fish and 
squid available at the surface in the area. 
If one is to use sea birds as samplers of their 
marine environment, it is important to have 
some understanding of the part which they 
play in the ecology of the surface layer of the 
sea. All the terns (including "noddies”) con- 
sidered here obtain their food within a few 
centimeters of the surface of the sea; boobies, 
Sula spp., tropic birds, Phaethon spp., and some 
shearwaters (Puffin us spp.), can penetrate a 
little deeper. Thus the terns should be sampling 
the fauna characteristic of the surface film, or 
"neuston.” In fact, only one of the species for 
which we have data — the Blue-grey Noddy, 
Procelsterna cerulea — has a diet with a composi- 
tion conspicuously similar to hauls made with 
the "neuston net” in the Indian Ocean (David, 
1965) : both contain large proportions of water 
striders (Halobates) , pontellid copepods, and 
very small fish larvae. Among the other terns, 
only A. tenuirostris sometimes took many small 
fish larvae ; and small crustaceans and Halobates 
were absent from the diet of all the larger 
species. Although there is little information 
available as to how many larger animals are 
normally present within a few centimeters of 
the surface during the day, observations suggest 
that, except for Procelsterna cerulea (and per- 
