Náttúrufræðingurinn - 1954, Page 26

134
NÁTTÚRUFRÆÐINGURINN
As the great skua very rarely nests on bare sand or gravel its distribution and
breeding density is greatly affected by the existence and extent of vegetation, and
ns a rule the nests are only found where there is some vegetation of one of the
three main types mentioned above.
The latter half of May is the main laying season of the great skua in Iceland.
May 9 is the first egg date ever recorded, but in an average season laying to any
extent does not begin until about May 20, and in exceptionally bad seasons, not
until the end of May or the beginning of June. Thus the great skua nests a little
earlier than the arctic skua, but somewhat later than the great black-backed gull.
As mentioned above these three species often share the same habitat.
The nest of the great skua is a mere hollow in tlie ground. The diameter of
the nest hollow is 23—30 cm and the depth 4>/2—8 cm. It is mostly lined with
insignificant quantities of dry grasses or moss. The normal clutch consists of
2 eggs. Clutches of 1 egg only are veiy rare and a clutch of 3 eggs has never
been recorded from Iceland. Hálfdan Björnsson of Kvisker in öræfi has twice
ascertained the incubation period of the great skua, and in both cases it was found
to be 28 days. He also found out that 2—3 days elapse between the laying of the
eggs. In some parts of Iceland young great skuas have been caught for human
consumption. In such cases the birds were taken in the 15th week of summer
(at the end of July), or shortly before they were fledged.
In spring the great skua starts visiting the nesting areas in the latter half of
March. According to 12 years observations by the Kvísker brothers the arrival
dates of the great skua on Breidamerkursandur have been from March 11 to
April 1. It should, however, be kept in mind that these observations only refer
to the first bird or birds seen and that a considerable time may elapse from that
date until the whole breeding population has returned to the breeding-grounds.
In August, when the young are fledged, the skua begins to desert the nesting
areas and moves out to sea, but scattered birds may nevertheless be seen on the
breeding-grounds until the first week of September, and fledgelings have even
been found on Breidamerkursandur up to that time.
It is known that the great skua, like the arctic skua, moves out to sea in winter,
although the great skua is more often seen near land at that time of the year
than the arctic skua. The winter quarters of the great skua are said to be in the
North Atlantic between 60°N and 23*4°N. Great skua fledgelings have been band-
ed in some numbers in Iceland in recent years. Only 5 banded birds have been
recovered in winter, all of which were in their first winter. One of these birds
was shot off Newfoundland (Horse Islands) in October, the second in the English
Channel (off Montmartin sur Mer, France) in November, the third off the Bel-
gian coast (30 km N. of Blankenberge) in December, the fourth in December in
tlie SE. corner of the Bay of Biscay (near the French-Spanish border), and the
fifth was found dead in January in the Isle of Skye off the west coast of Scotland.
Summer recoveries of banded birds are more numerous. Of birds in their first
summer one was shot off Newfoundland (Strait of Belle Isle) in July. Of birds
in their second summer 2 were shot off SW. Greenland in Aúgust and 3 off
N. Iceland in June—August. Of birds in their third summer 1 was shot off SW.
Greenland in August and 6 were shot off N. Iceland in May—July, one of the
latter being shot near the breeding colony where it had been banded. Of birds in