Náttúrufræðingurinn - 1953, Blaðsíða 37

ISLENZKIR FUGLAR VII
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Vogelwelt Islands, Berlin 1903). The size of the Grimsey colony has been subject
to considerable fluctuations since that time. In June 1934 the colony suffered seri-
ously through dislodgement of material from the cliff fase as a result of the Dalvik
earthquake. In spite of this 43 nests were found on Grímsey in 1939 (Vevers et al.).
After 1939 dislodgement of material from the cliffs occurred repeatedíy, and this
may he the main reason for the dwindling of the Grímsey colony which continued
until only 3 pairs were left there in 1946. Since that time no gannets have
bred on Grimsey. Finally it should he mentioned that on the eastem side of Stór-
höfdi on the island of Heimaey in the Vestmannaeyjar there is a place called Súlu-
krókur (gannet coraer). This is a long and broad ledge in the cliffs and many
things warrant the assumption that it was occupied by gannets at some time or
other, although evidence for this cannot be produced.
With exception of the small and comparatively recent colony in Raudinúpur
all the gannet colonies in Iceland are to be found on uninhabited, remote and
rocky islands. In such places the gannets may occupy the top plateau of the islands
(cf. Eldey and Súlnasker) or they may breed on ledges or shelves in precipitous
cliffs. The nest is a substantial structure. It is made of sea-weeds as well as bits of
terrestrial vegetation in so far as this is available. Feathers are also found in most
nests and sometimes also small sticks of wood or any other kind of flotsam. The
nest is about 40 cm in diameter at the bottom, but somewhat narrower at the top.
The height of the nests varies a great deal, since annual additions are made to old
nests. The average height is 30—40 cm, but the highest nests can reach the height
of 1 m or more. On the other hand birds nesting for the first time can boast of
only very rudimentary nests. In dense gannet colonies the nests are often so close
to one another that they are only separated by narrow channels or hollows where
the excrement of the birds accumulates. Owing to the high precipitation in Iceland
the excrement cannot accumulate from year to year and this excludes the forma-
tion of guano deposits. In heavy rain the excrement tums into mud, and on Eldey
and Súlnasker mudstreams may then be seen flowing along rifts or depressions
over the brinks of the cliffs.
The gannet lays only one egg. In Iceland the first gannets start laying at the
end of March but April is the main breeding season, although many birds do not
lay until May. The result is that in the same gannetry we can find eggs as well
as young birds of very varying maturity at the same time. In Iceland the last
nestlings will not make for the sea until October, probably in the first half of this
month, and the adult birds leave the breeding stations about the same time. As far
as is known, gaimets are rarely, if at all, seen in inshore waters from late
October until December, but after the middle of December gannets will appear
again in the waters around the Vestmannaeyjar and at the south-west coast of the
mainland, and in February (and possibly earlier) gannets start visiting the breed-
ing stations. In spite of this, adult gannets can hardly be regarded as migratory
birds in Iceland, even though they most probably stray far in search of food
outside the nesting season. Thus two adult gannets banded in the Súlnasker gan-
netry in June were caught in January of the following year off the Norwegian
coast near Bergen. Juvenile birds, on the other hand, especially birds in their first
year, are probably mostly migratory, but nothing is known about their move-
ments and winter quarters.
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