Birdwatch UK October 2017

http://www.birdguides.com/birdwatch Birdwatch•October 2017 45

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MIGRATION

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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