Náttúrufræðingurinn - 1957, Síða 63

ÍSLENZKIR FUGLAR XIV
217
ihat the arctic tern does not attain sexual maturity until 3 years old ancl prob-
ably the majority of the birds lead an oceanic life unil then. However, arctic
terns in first-summer plumage (Sterna portlandica Ridgway) witli white fore-
head, black bill and legs, and white below, have occasionally been met with in
colonies of the arctic tern in Iceland. As a rule these have only been odd strag-
glers, but in late June and July, 1954, I encountered flocks consisting of up to
100 individuals oí such birds near small terneries at the mouths of some glacial
rivers on the low-lying and exposed soutli coast of Iceland. These one year old
birds used to rest in tight flocks on shingle banks in the outskirts of the colonies.
When the colonies were disturbed they would take wing together with the adult
breeding birds and would fly screaming among theni above the breeding-
grounds, but when the birds descended again the one year old birds would
separate from the adults ancl alight in a closed flock a little away from the
main colony. It should be stated, however, tliat one year old birds have never
been proved to breed in Iceland. Assertions to the effect that birds in first —
summer plumage liave been found breeding elsewhere are in my opinion quest-
ionable, or do at least require verification.
The food ol the arctic tern in Iceland consists chiefly of small fish, plank-
tonic invertebrates (notably euphausids), insects and insect larvae, and earth-
worms. Marine fishes taken include sand-eels (Ammodytes), capelin (Mallotus
villosus), and juvenile stages of herring (C.luþea harengus), coalfish (Pollachius
virens) and several otlier species. In fresh-water tlie three-spined stickleback
(Gasterosteus aculeales) is the most important food fisli whereas juvenile stages
of trout, char and salmon are of less importance. As a consequence of the wide
inland distribution of the arctic tern in Iceland insects and other terrestrial
invertebrates no doubt play a more important róle in the diet of arctic terns in
Iceland than in many other countries. Tliis is borne out by the fact, that practi-
cally everywhere in lceland arctic. terns arc regularly observed in large numbers
hawking for food over meadows, marshes and moorlands. Cultivated areas in
particular appear to attract the terns, probably because of their rich insect íife.
Thus the terns habitually gather in large numbers over freshly mowed hay mea-
dows and they eagerly hunt'for earthworms in potato and turnip fields.