Náttúrufræðingurinn - 1954, Blaðsíða 42

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NÁTT0RUFRÆÐINGURINN
many lakes in other parts of the country great blackback colonies are frequently
found on islands in the lakes. The great blackback, however, has not been found
breeding up to this time in the vegetation oases of the Central Highlands, although
there are many indications that it is now extending its range to these areas, pro-
bably due to increase and overcrowding in other parts of the country. In recent
years, for example, the great blackback has been observed in considerable numbers
at Veidivötn on Landmannaafréttur, and in 1951 in Thjórsárver at Hofsjökull the
great blackback was present in numbers throughout the summer, although breeding
could not be proved.
Throughout Iceland the great blackback may be found breeding singly, although
in many areas colonial breeding occurs quite commonly. The colonies of greatest
density occur on islands on lakes and rivers, and in the largest Icelandic colonies
the number of breeding pairs may reach a few hundred. The nest of the great
blackback is a bulky structure and, for the mots part, surprisingly well constructed.
The nest hollow is 25—30 cm in diameter and 7—9 cm deep. The nest is made of
moss, grasses and seaweed, depending upon which material is most readily obtain-
able in the immediate vicinity of the nest. In most nests a few large feathers are
found with the above-mentioned materials. The clutch consists of 3 eggs, very rarely
only of 2 eggs. The eggs are laid in May. In an average season complete clutches
are usually not found until the middle of that month. In climatically favourable
areas and in good seasons, egg-laying may begin earlier, even at the end of April,
but in more unfavourable districts and in colder seasons egg-laying may be con-
siderably delayed, even until the end of May.
The great blackback is doubtlessly for the most part a sedentary bird in Iceland.
Rut recoveries of ringed birds have shown that it is more or less subject to dis-
persal movements within the country itself, and that a part of the Icelandic popula-
tion leaves the country in winter. This is proved by the fact that, of great black-
backs ringed as young in Iceland, 6 have been recovered abroad, of which 4 in the
Faeroe Islands, 1 in the Outer Hebrides, and 1 in Northern Ireland. All these
were immature birds in their first or second winter. In winter the great blackback
keeps mainly to the coast, but also to some extent to ice-free lakes and rivers. The
great blackback is, moreover, a bird of inshore waters throughout the year, and it
is rarely found beyond the limits of the shelf round Iceland.
The great blackback is an omnivorous bird, and its food also varies considerably
with the locality and the time of the year. In spring and summer it preys to
a great extent upon the eggs and young of other birds, and during this time it
does a great damage in eider-duck and other bird colonies. Adult birds, however,
are rarely attacked unless they are wounded. The great blackback is, however,
occasionally parasitic on diving ducks such as the eider and the red-breasted mer-
ganser. In spring it also takes considerable numbers of lump-fish in tidal pools and
shallow water. Salmon and trout are also taken from the shallow parts of lakes
and rivers and from shallow estuaries. Smaller marine fishes such as capelin, her-
ring, sand-eel, juvenile stages of the coal-fish etc., which can be obtained from the
surface, are also important as food. Rats and mice are occasionally taken and, in
inland districts where other food is scarce, young lambs are sometimes attacked.
Certain marine invertebrates such as crustaceans, molluscs, echinoderms (sea-ur-
chins), and worms also play an important róle in the diet of the great blackback.