http://www.birdguides.com/birdwatch Birdwatch•October 2017 45
❯❯
MIGRATION
❝
The last few
years have seen
increasingly severe
droughts in Spain,
sometimes with
associated wild res
and the loss of
vegetation that could
hold the caterpillars
thought to be used
by migrating Common
Cuckoos to fuel their
journey
❞
that extends from the Democratic of
Congo in the east to Gabon in the west.
The presence of this route had been
predicted from the small numbers of
ringed Common Cuckoos to have been
recovered from around the eastern end
of the Mediterranean.
Birds using the south-easterly route are
joined on their African wintering grounds
by others that leave Britain on a south-
west heading, fl ying through France
and Spain and crossing the western end
of the Sahara before turning east. The
presence of this western route was totally
unexpected, a fi nding made all the more
interesting by the subsequent discovery
that all of the cuckoos use the western
course during spring migration that sees
them return north.
Trouble in Spain?
The tracking data has enabled Dr
Hewson to assess whether the levels of
mortality associated with the di erent
routes vary, perhaps su ciently enough
to explain why some British populations
are faring worse than others. He found
that survival rates are signifi cantly lower
on the fi rst stage of the western route
- from British breeding areas to the
southern edge of the Sahara – than on
the easterly heading. This is despite the
TOBY SMITH (WWW.BTO.ORG)
MIKE TOMS (WWW.BTO.ORG)
fact that the former is some 12 per cent
shorter up to this point.
The last few years have seen
increasingly severe droughts in Spain,
sometimes with associated wildfi res and
the loss of vegetation that could hold
the caterpillars thought to be used by
migrating Common Cuckoos to fuel
their journey. Other developments in
the region – such as changing land use
or increased predation pressure – may
also be playing their part.
Interestingly, when Dr Hewson
looked at the proportion of birds using
the western route from each of his nine
study areas, he found that, while all of
the birds from upland sites in Wales
and Scotland used the eastern route,
those from the remaining locations
used a mixture of the two routes. He
also found that the proportion of birds
using the western route was correlated
with the degree of local population
decline evident within the Breeding
Bird Survey data. This provides good
evidence that conditions encountered
during migration can have an impact
on breeding populations, and that
the use of the western route has
contributed to the declines seen in
some populations.
Dr Hewson used tracking data
collected between 2011 and 2014, since
when drought conditions have hit the Po
Valley in northern Italy, a crucial stop-
over site on the eastern route. This raises
an important question about the levels
of mortality occurring during autumn
Top: BTO Senior Research Ecologist Dr Chris Hewson has been using satellite tags to track the
movements of British cuckoos, revealing for the fi rst time that the birds use two different routes to
reach the same wintering area.
Above: wing length is one of the standard measures taken by bird ringers, providing an indication of
the structural size of the bird.
1710 p044-047 lines of flight FIN.indd 45 15/09/2017 14:38