The Migration of BirdsCoward, T. A. (Thomas Alfred)
Science
The Migration of Birds
Coward, T. A. (Thomas Alfred)
Birds -- Migration
Compare this with Cordeaux's notes of another goldcrest immigration,
this time to the Lincolnshire coast (23). On October 13th the wind was
north to north-east in the afternoon, light but increasing in force, the
weather clear and bright--a few birds arrived. They had started under
favourable circumstances. Shortly after midnight on the morning of the
14th, the wind got full east, with quite half a gale and heavy beating
rain, continuous to the morning of the 16th; the nights were very dark.
"During this time the immigration was immense," and most of the birds
were goldcrests. Cordeaux's idea that these were not normal immigrants
but birds which were passing probably from north-east to south-west,
when the easterly gale caught them, is probably correct.
I have referred to birds starting at a high elevation. Service says
that in normal departure from the Solway, most birds mount to a high
altitude, but "a strong beam wind will bring the birds--even those of
strongest power--down to 200 to 500 feet of the surface, and it is
interesting to see whole flocks with heads turned almost completely to
wind, and yet travelling along at nearly their normal speed, at right
angles to their position" (46). Mr Tomison mentions rooks, daws and
hooded crows driven to Sule Skerry by south-east winds in March, leaving
two days later in a westerly gale. They, at any rate, did not object to
a strong wind which was in the right direction.
I have mentioned Mr F. J. Stubbs' paper on the "Use of Wind" (50),
and I believe that there is much more in it than is actually proved by
low-level observations. I doubt if birds always intentionally make use
of strong winds, currents which would carry them for great distances
at a considerable speed, but the preliminary ascent may be to search
for these currents. Cyclonic and anticyclonic winds, even when at
an altitude of some thousands of feet, would carry them easily, and
probably it is the wind-borne individuals, parties, or even hosts,
which drop for a refuge to the first island they see when carried far
from their migratory path. They are carried rather than drifted from
their pathway, borne in the moving current whether they wish it or
not. Provided that the cyclonic winds are fairly steady in direction
and force, sweeping round and inwards towards their centre, we may in
imagination trace the pathway of our so-called lost wanderers to far
distant islands; without many more upper-air observation stations, we
cannot actually prove the route.
Public-domain text, read in full here on John Shaqi.
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