Sea Bird Food and Ocean Surface Fauna — Ashmole and Ashmole 
3 
may be caught, the methods and zones which it 
uses in feeding, and the size of the animals 
which it obtains. 
EVALUATION OF THE BIRD SPECIES 
Of the eight bird species whose food was 
sampled on Christmas Island (Table 1), four 
would not be very suitable for marine biological 
investigations. It is time consuming to obtain 
samples from the Christmas Island Shearwater, 
Puff in-us, nativitatis Streets, the Phoenix Petrel, 
Pterodroma alba (Gmelin), and the Red-tailed 
Tropic-bird, Pbaetbon rubrtcauda Boddaert, and 
the number of identifiable fish which can be 
expected in 100 samples (Table 1) is rather 
low for all these species. The Blue-grey Noddy, 
Procelsterna cerulea (F. D. Bennett), provides 
large numbers of items per sample, but the 
populations on many islands are rather small, 
and the species is not widely distributed. Fur- 
thermore, it feeds close inshore on very small 
items which could probably be sampled more 
efficiently with a neuston net (David, 1965) 
towed behind a small boat. The other four spe- 
cies studied — the Sooty Tern, Sterna fuscata 
Linnaeus, the Brown Noddy, Anous stolidus 
(Linnaeus), the Black or Lesser Noddy, Anous 
tenuirostris (Temminck), and the White or 
Fairy Tern, Gygis alba (Sparrman) — all have 
certain advantages for regular study. Salient 
numerical details are given in Table 1. 
Sterna fuscata (Sooty Tern) 
This is the most .abundant of all tropical sea 
birds, breeding in enormous colonies on islands 
in the tropics around the world (see Ashmole, 
1963 for details of its breeding distribution and 
breeding seasons). The eggs are laid on the 
ground in the open, and sometimes there are 
as many as five nests per square meter. The 
birds may desert whole areas if they are dis- 
turbed too much at the start of nesting, but 
later they sit tightly and may be caught with a 
hand net. However, incubating birds rarely re- 
gurgitate when caught, and the only way in 
which large numbers of samples can easily be 
collected is to catch adults (with a long-handled 
hand net, or mist nets) as they arrive to feed 
their chicks, especially in the late afternoon, or 
to catch chicks shortly after they have been fed. 
Before breeding starts, a few regurgitations can 
be obtained by catching birds at night when 
they are roosting on the ground, but the samples 
are generally small and largely digested. On 
most tropical Islands the species has one fairly 
short breeding season, but on Christmas Island 
and a few other central Pacific Islands there are 
two breeding seasons each year. 
Sterna fuscata catches its prey either while 
flying or by plunging to the surface, but it prob- 
ably hardly ever submerges completely. It has 
been recorded as feeding at night (Gould, in 
press), but this probably occurs only when the 
moon is nearly full. It is an oceanic species 
capable of feeding hundreds of miles from land, 
even when breeding. However, the actual dis- 
tance traveled regularly probably varies con- 
siderably from colony to colony. 
Our 242 samples were mostly from adults 
feeding chicks. They contained, on the average, 
5.6 items each, of which 60% were fish, 40% 
squid. On the whole, the samples were in good 
condition, and 79% of the fish were identified 
to the family level. This means that about 266 
identifiable fish could be expected per 100 sam- 
ples. The identified fish belonged to 21 families, 
the ones which occurred most regularly being 
Exocoetidae and Scombri dae, Gempylidae, Ser- 
ranidae, and Emmelichthyidae. The identified 
squid were nearly all Ommastrephidae. Of the 
fish, 93% were between 2 and 10 cm in length 
(measured to the base of the tail), while 93% 
of the squid were between 2 and 8 cm in 
mantle length. We obtained sufficient data from 
this species to suggest that important seasonal 
changes exist in the availability of certain fish 
families, but a more intensive sampling program 
would be necessary to demonstrate such changes 
convincingly. 
In summary, the advantages of this species 
are that it Is available in great numbers, and 
samples can easily be obtained; the samples .are 
often in good condition, and contain a high 
proportion of identifiable items ; Scombridae 
are especially well represented in the samples; 
and the feeding range of the species is such 
that it provides samples from a large oceanic 
area. However, one cannot determine precisely 
the area from which each sample comes, and 
it is not easy to obtain many samples at times 
when the birds are not breeding. 
