At this time of the year, many small birds will be getting ready to
migrate south for the winter. Preparation is largely centred on the need to
build up sufficient fat reserves to fuel the journey ahead, the amount of fat
deposited varying in relation to the length of the distance to be covered.
Thanks to the efforts of bird ringers we know a lot about the different
strategies adopted by migrating birds. Some, for example, complete their
journey in a single hop, while others break it up into a series of smaller
flights each broken by a stopover to take on fuel. Many small birds will, for
example, stop before crossing the Mediterranean, fattening up on its northern
shores.
Now you might think that Swallows, being aerial feeders, would not need
to fatten up. They can, after all, feed as they go, snacking on the aerial
plankton of flies and other invertebrates with which they share the sky. It seems, however, that this is not the
case and that our Swallows also fatten up ahead of their journey. The latest
research on this subject is based on some work with which I, as a bird ringer,
was involved a few summers ago – you may even remember me discussing it in one
of my previous columns. We’d spent a run of summer evenings netting Swallows at
South Lopham, as the birds came into roost each evening in the reed beds there.
It was a wonderful way to spend the evening, hearing the birds approach and
then watching them drop down to roost. There was even the occasional sighting
of a Hobby, it too fattening up ahead of its migration – though this time
fattening up on the Swallows and martins! We were not working alone and it is
only now, seeing the report in which the research was published, that you
realise just how many other bird ringers were spending their summer evenings in
a similar fashion, at other reed beds up and down the country.
As well as ringing the birds, we also took various measurements, some of
which revealed the quantity of fat reserves being laid down. The results show
that our Swallows fatten up in southern Britain, taking on sufficient reserves
to allow them to travel to the Continent where, according to the work of
colleagues overseas, they then take on more reserves in preparation for
crossing the Mediterranean and the Sahara of northern Africa. It is amazing to
think that our efforts have contributed, in some small way, to increasing our
understanding of how these wonderful birds migrate, and the strategies that
they use to undertake such an amazing journey to their wintering grounds in
South Africa.
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