44 Birdwatch•October 2017 http://www.birdguides.com/birdwatch
MIGRATION
EDMUND FELLOWES (WWW.BTO.ORG)Lines
of fl ight
Recent research has thrown light
onto the migration routes of
Common Cuckoo. The BTO’s Mike
Toms takes a look at the
sometimes surprising ndings, and
how art is meeting science to bring
the information to the masses.I
n order to experience a reedbed in the
way that a Reed Warbler or Common
Cuckoo does, you fi rst need to slip
on a wetsuit or a pair of chest waders.
Only then can you enter their strangely
vertical world to discover something of
the evolutionary arms race that exists
between Common Cuckoo and its host.
Thanks to the work of researchers
like Professor Nick Davies, whose
Wicken Fen study continues to deliver
new insights into CommonCuckoos and Reed Warblers, we now
have a good understanding of this
particular component of the cuckoo’s
life cycle, but what about the rest of it?
What happens once the birds have left
our shores?
Our knowledge of this summer
visitor is far from complete; a fact made
all the more alarming as breeding
Common Cuckoo numbers have
declined over recent decades. Figures
from the Breeding Bird Survey
reveal that the species’ British
population fell by 43 per cent
between 1995 and 2014.
The pattern of loss has not
been the same across the whole
of Britain. It is worst in England,
where the species has slumped
by a huge 68 per cent, followed
by Wales, with a decline of
18 per cent. In Scotland,
however, Common Cuckoo
populations actually increased
by a third over the same period.
These regional di erences
are particularly evident in the
change in relative abundance
map in the Bird Atlas 2007-11.Understanding
the decline
Work on British cuckoo
populations suggests that the
reasons behind the decline across
southern Britain may lie elsewhere,
either on the wintering grounds
or during the long migratory
journeys that these birds undertake.
Establishing whether or not this
is the case has had to wait until
suitable tracking devices have become
available, something that has only
happened relatively recently.
Since 2011, British Trust for
Ornithology (BTO) Senior Research
Ecologist Dr Chris Hewson has been
fi tting tiny satellite tags, weighing just a
few grams, to male Common Cuckoos
in an attempt to understand what these
birds do once they leave our shores.
These devices have enabled Dr Hewson
to chart, for the fi rst time, the migration
routes and stop-over sites used by
cuckoos from di erent parts of Britain.
The fi rst and rather surprising
fi nding to emerge from an analysis
of the tracking data is that British
cuckoos use two di erent migration
paths to reach the same wintering
area in Africa. Some birds use a route
that takes them south-east to Italy and
the Balkans, before they turn south,
crossing the Mediterranean and then
the Sahara to reach a wintering areanew insights into Common from the Breeding Bird Survey
reveal that the species’ British
population fell by 43 per cent
between 1995 and 2014.been the same across the whole
of Britain. It is worst in England,
where the species has slumped
by a huge 68 per cent, followed by a huge 68 per cent, followed
by Wales, with a decline of
18 per cent. In Scotland,
however, Common Cuckoo
populations actually increased
by a third over the same period.
These regional di erences
are particularly evident in the
change in relative abundance
map in theMain photo: regional differences
in the declines seen in Common
Cuckoo populations have been
linked to migration route.
Left: Common Cuckoo relative
breeding abundance 2008-11.
Map reproduced from the Bird
Atlas 2007-11, a joint project
between the BTO, BirdWatch
Ireland and the Scottish
Ornithologists’ Club.1710 p044-047 lines of flight FIN.indd 44 15/09/2017 14:38