Náttúrufræðingurinn - 1955, Side 42

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NÁTTtJRUFRÆÐINGURINN
the impression that the glaucous gull in Iceland is not as predatory as the great
black-back. In liigh-arctic regions, on the other hand, where the great black-hack
is not found, the glaucous gull is said to be a bold and agressive predator. If such
a geographical difference in feeding habits of the glaucous gull actually exists, it
could be explained either tlirough scarcity of other food in the high-arctic areas or,
probably more likely, through the strong competition witli the great black-back in
Iceland, which may have caused some change in its feeding habits. This would be
an interesting subject for further investigation.
It may be assumed that the glaucous gull, being an arctic species, was more
common and more widely distributed in Iceland during the pleistocene glaciation
than at present. The great black-back, on the other hand, as a species adapted to
milder climatic conditions, was probably not able to exist in Iceland at that time.
After the close of the ice-age the great black-back must have moved northward to
colonize Iceland where it probably spread rapidly. Now the glaucous gull and the
great black-back are closely related species of similar size and with similar ecolo-
gical adjustment. They are, therefore, competing species which usually inliabit dif-
ferent areas. But where their ranges overlap as in Iceland a strong competition
develops between the two species. The restricted and peculiar distribution of the
glaucous gull at present in Iceland is, in my opinion, to be ascribed mainly to this
competition and thus only indirectly to the climatic improvement following the
last ice-age.
The surprising fact that we find a relic population of an arctic species like the
glaucous gull almost exclusively confined to the sheltered and climatically favoured
districts round Breidafjördur is, as I think, to be explained through the existence of
the innumerable islands in the Breidafjördur archipelago. The glaucous gull, it is
true, nowhere nests on these islands which, on the other hand, support a very large
population of great black-backs. If low islands, islets, and skerries are available
the great black-back prefers to nest in such habitats rather than on cliffs. The
multitude of islands in the Breidafjördur archipelago provides the great black-back
with such a wide range of optimal nesting habitats that the coastal mountains
round the bay have been left unoccupied. This is probably the reason why the glau-
cous gull has been able to hold its ground there and not to any extent in similar
habitats in other parts of the countiy where the great black-back is nesting abun-
dantly, This indicates that competition for nesting sites is the decisive factor in the
competitive relationship of these two species. It furlhei-more strongly supports the
assumption that the small population and restricted distribution of the glaucous gull
in Iceland is to be regarded as a result of the competition with the great black-back.
In this connection it may be mentioned that, in arctic areas where the great black-
back is not found, the glaucous gull is also found nesting on low-Iying coastal islands
and locally even on inland lakes.
There is some indication that the recent amelioration of climate in Iceland and
the siinultaneous and very pronounced increase of the great black-back are affect-
ing the present status of the glaucous gull. This is bome out by persons living in
the neighbourhood of the glaucous gull colonies in the Breidafjördur area. They
unanimously maintain that the glaucous gull has been decreasing seriously in the
last decades. They ascribe this decrease mostly to the breeding of an increasing