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BOX-GIRDER
SWALLOWS We
used to call them "Swallows" but, as there are a
dozen or so swallow species in their African wintering grounds,
the "politically correct" name for our species is
now "Barn Swallow". As the names imply, our Barn
Swallows are more rural than House Martins, preferring to
nest on a ledge in an outbuilding or barn with more open cattle
pastures around for feeding. In
an interesting piece of evolutionary adaptation some Swallows
have established a colony of 10 - 15 nests underneath the
bridge that carries the A590 Dalton Bypass over Thwaite Flat
- hence Box-girder Swallows not Barn Swallows!!! The box-girders
of the bridge afford ample ledges and shelter. However, every
time a vehicle hurtles across the bridge there is a loud thud
as it passes over the expansion plates. Neither the noise
nor the vibration seems to deter the birds, although lesser
mortals like us would get a thumping headache and their homes
would probably collapse if they lived with this twentyfour-seven. In
fact the birds arrive back in late April/early May after spending
the winter on the southern tip of Africa with 300 million
or so other Barn Swallows from all over Europe. Even with
the traffic, Thwaite Flat must seem like a peaceful haven
compared to the noise experienced at a night-time roost there!
In
another piece of adaptation to man made structures, Swallows
characteristically rest on telegraph and electricity wires,
which provide an unimpeded access that suits their swooping
flight. Even the young seem to have this habit built in, as
they often line up on wires after fledging, waiting to be
fed (see photo below): These
are tough little beasts anyway. At five months old they begin
an incredible 12000 mile round trip that sees them travel
across the English Channel, the Pyrenees, the Sahara Desert,
Equatorial Africa and down the east coast of South Africa
...then back again two months later. Each night a roost site
has to be found and an incredible variety of predators and
weather conditions must be overcome. During the four days
it can take to cross the Sahara an ascent to 3 miles may be
needed in order to spot the next oasis fifty miles away. Fewer
than 20% of first year birds make it back again but, of those
that do, many find their way back to the same nest site each
year for up to ten years.
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