6 
PACIFIC SCIENCE, Vol. XXII, January 1968 
are from inshore waters; the small size of its 
prey would make study of its food an excellent 
complement to that of one of the larger species, 
for instance A. stolidus. Its main disadvantage 
is the difficulty of identifying many of the fish 
fry. 
Gygis alba (White or Fairy Tern) 
Gygis alba, another widespread tropical spe- 
cies, generally is less abundant than the preced- 
ing three species, but many tropical islands have 
populations of a thousand or more birds. The 
nests are dispersed, either in forks or hollows in 
branches of trees, on the tops of coral blocks, 
or on cliff ledges where these are available. 
G. alba differs from the other species in that 
the adults rarely regurgitate, and they carry 
food for the young in their bills. Hence the 
technique for obtaining food samples is different. 
The birds usually come quite close to a human 
intruder, even when carrying food, and may 
often be caught with a long-handled net; the 
food items dropped by the birds as they are 
caught can then be retrieved from the ground. 
Our samples were mainly obtained from adults 
in this way, although we also collected regurgita- 
tations from juveniles and from adults. 
On Christmas Island G. alba breeds at all 
times of year. In many other areas, however, 
breeding is more seasonal and, since the individ- 
uals leave the colonies when they have finished 
breeding, it would be impossible to obtain 
samples at all times of year. 
This species catches most of its prey in flight, 
and there is some evidence that it feeds exten- 
sively in the half light at dawn. It sometimes 
feeds close inshore, and most of the prey carried 
back to the young in the bill are probably caught 
fairly close to the colony; however, the species 
sometimes ranges hundreds of miles from land. 
Among our 152 samples, those carried in 
the bill contained, on the average, 1.3 items 
each, while regurgitations contained 4.3 items. 
Fish made up 59% of the items, the remainder 
being squid. The samples were generally in 
excellent condition, and 87% of the fish were 
identified to the family level. Collecting only 
food carried in the bill, one could expect to 
obtain approximately 67 fish identifiable to 
the family level per 100 samples. However, 
all of these would be in excellent condition, so 
that further identification would be much easier 
than in most of the other birds. The fish which 
we identified represented 22 families, 5 of 
which were not found in the food of any of 
the other bird species. Blenniidae occurred most 
regularly, followed by Exocoetidae, Myctophi- 
dae, Scombridae, Gempylidae, and Gonostom- 
atidae. Squid of the families Ommastrephidae 
and Enoploteuthidae were identified. Of the 
fish, 96% were less than 8 cm in length; 
the samples carried in the bill contained a 
lower proportion of small fish than did the 
regurgitations. Of the squid, 93% were between 
2 and 6 cm in mantle length. 
This species ate far more squid during one 
of our sampling periods than at other times, 
and during this same period the squid taken 
included Enoploteuthidae ( Abralia sp.) which 
were not found in samples obtained in the earlier 
part of the study. 
The main advantages of this species are the 
excellent condition of the items carried in the 
bill and the fact that a wide variety of prey 
are eaten. However, it is difficult to obtain 
many samples, since usually the populations are 
not very large, and some of the birds are too 
wary to be caught when carrying fish, so that 
a quantitative study would probably have to 
depend primarily on other species. 
Other Species 
The species just discussed are those with 
which we have most experience, but other spe- 
cies also might be used for sampling, and we 
have tried to indicate some of the criteria v/hich 
should be used in deciding whether a given 
species would be appropriate. The only large 
species included in our study was Phaethon 
rubricauda (weights are included in Table 1), 
which takes large prey, but from which it is 
generally not easy to obtain many samples. 
The obvious additional candidates, available on 
many tropical islands, are the various species of 
frigate birds, Fregata spp., and boobies, Sula 
spp. We would reject the frigate birds on the 
grounds that they obtain some of their food by 
piracy, which complicates interpretation, and also 
the food generally is in poor condition. Of the 
three widespread tropical boobies, the Brown 
Booby, Sula leucogaster (Boddaert), is mainly 
an inshore feeder, is rather difficult to catch, and 
