Media playback is unsupported on your device Media caption How the team tracked the cuckoos

The sound of the first cuckoo in spring is a familiar one in the British countryside.

But the summer visitor is in decline and, according to a new study, its migratory habits may be to blame.

Scientists have tagged birds leaving the UK and believe they take two different routes on their journey to spend the winter in Africa.

Surprisingly, survival is lower on the shorter route via Spain, they report in the journal, Nature Communications.

And this suggests that migration - as well as other factors such as loss of farmland and insect food - may be to blame for the cuckoo's decline.

More than half of cuckoos in the UK have been lost over the past 20 years, according to the British Trust for Ornithology (BTO) in Norfolk, which led the research.

"That pattern of starting in the same place but taking two very different routes to get there has not been seen before in any birds, to the best of our knowledge," said lead researcher Dr Chris Hewson.

"We need to understand the full annual cycle of a migratory bird in order to understand its population decline."

Facts about the common cuckoo

Image copyright ROGER WILMSHURST/SCIENCE PHOTO LIBRARY Image caption Cuckoos are known for taking over other birds' nests

British summer visitor from Africa has a slim body, long tail and pointed wings

Male is blue-grey in colour, apart from a black-and-white-striped belly

Can be found in a variety of habitats but is most often seen in reed-bed, moorland, woodland or farmland

Female cuckoo is known for picking another bird's nest, then removing an egg, only to replace it with one of its own

Hatched cuckoo chick then pushes all the other chicks out of the nest to get food from the host parents

Cuckoos can be seen throughout the UK, but are especially numerous in southern and central England

Adults arrive in late March or April and depart in July or August, with young birds leaving a month or so later

There are 13,000 to 26,000 pairs in Britain, but numbers appear to have been falling in recent years

Populations in Scotland are stable, with numbers falling most dramatically in England, followed by Wales

Source: British Library/RSPB/BTO

Since 2011, the BTO has been satellite-tracking cuckoos to study their migration patterns when they leave the UK.

Working with the University of Copenhagen, the charitable research institute used satellite tags to track 42 male common cuckoos from the UK population during more than 50 autumn migrations.

The researchers found that birds from declining populations were more likely to migrate to winter breeding grounds in central Africa along a western route (through Spain) than along an eastern route (via Italy and the Balkans).

The higher mortality occurred before reaching the harsh environment of the Sahara desert, despite the fact that the western route is about a tenth shorter at this point.

The scientists think birds may have encountered challenging drought conditions in Spain.

Alternatively, they may have been deprived of insect food such as hairy caterpillars before leaving the UK, leaving them with lower fat stores for their hazardous journey.

Migratory bird species are increasingly threatened around the world due to factors including climate change, habitat change and habitat loss.

Migratory birds 'lack world protection'

Understanding where mortality occurs during their annual cycles is therefore increasingly important, especially for long-distance migratory land birds, which show some of the steepest population declines, say the scientists.

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