Náttúrufræðingurinn - 1962, Blaðsíða 41

NÁTTÚRUFRÆÐINGURINN
135
SUMMARY
Drift of migratory birds
by Jónas Jaliobsson
lcelandic Weather fíureau, Reykjavik.
In the fall af 1961 considerable amount of srnall migratory birds from
Scandinavia were found in Iceland. According to Dr. Finnur Gudmunds-
son, they appeared in ttvo swarms at the SE coast, the first one on the 8th
and 9th of October and the second one on the 24th.
Certain features of the weather situation these days must be responsible
for the erroneous track of these birds, whicli most likely are heading for Scot-
land from SW Norway. Weather charts are consulted. They show southeaster-
ly winds over the North Sea in both cases. On the Norwegian coast the
winds are lighter than out at sea, and off the Scotland coast they reach gale
force. The track of the birds is calculated, assuming their heading to be
from Stavanger to Petershead with flight speed of 20 linots. They start in
the evening, because they need the light liours to feed.
The first swarm is found to drift to the WNW, to pass just south of Shet-
land and run into fog and rain along a cold front near the Faeroes at 6
o’clock in the morning of the 8th (see chart 1). There they most likely lose
their orientation; and a good deal of them is bound to be carried with the
wind onwards to the coast of Iceland.
The second swarm doesn’t enter bad weather, but is blown off track to-
wards the NW. The wind direction changes rapidly due to fast movement of
lows at this time. Therefore, birds starting at different time in the evening of
the 23d will drift along different tracks (see chart 2). Birds leaving Stavanger
at dusk will hit the SE coast of Iceland in the afternoon of the 24th, where-
as the ones letting off at 21 GMT will continue their westward drift south
of Iceland. Later on, as the low west of Scotland moves north, tliey are carried
to the south and will possibly reach Scotland frorn the west after 36 hours of
flight.
Above examples sliow that migratory birds depencl on favorable winds and'
weather for safe flight across great bodies of water. Also it is evident that
on occasions thousands are bound to perisli.