When a migratory bird population is declining, what's the cause? Is it poor breeding success? Perils during migration? Decline on the wintering grounds? Or all of the above?
For common cuckoos in the U.K. there's a surprising answer. Though equally successful on their breeding and wintering grounds the birds have two fall migration routes, one more dangerous than the other. The population that takes the deadly route is declining.
Researchers attached tiny transmitters to 42 male common cuckoos (Cuculus canorus) across the U.K. and tracked them on migration from 2011 to 2014. All of the birds spend the winter in Central Africa but they choose two routes during fall migration. The eastern route goes east over Europe then south over Italy, the Mediterranean, and the Sahara. The western route is a shortcut over France, Spain and the western Sahara. Mortality was highest along the western route, especially in Spain.
Cuckoos that breed in Scotland and Wales take the long eastern route in the fall. Cuckoos nesting in southern England split with about 60% flying east, the remainder taking the west shortcut. Midlands and East Anglia cuckoos favor the western route.
Common cuckoos have a stable population in Scotland and Wales but a declining one in the midlands and eastern England. When the researchers matched migration routes with population trends in the Breeding Bird Survey and Bird Atlas they found a close correlation. Indeed, population decline in the common cuckoo is linked to their choice of migration route.
See the migration maps and read the 2016 study here at Nature Communications: Population decline is linked to migration route in the Common Cuckoo.
(photo of common cuckoo from Wikimedia Commons; click on the image to see the original)