But the most significant route to permanent settlement is the economic migration route.

BBC: In full: David Cameron immigration speech

Research will look at whether there are problems with habitat, either in the UK or in Africa, or on their migration route, he said.

BBC: Cuckoo's call becomes rarer in UK

Living in 14 villages along the Porcupine herd's migration route, some 7, 000 Gwich'in maintain a traditional culture linked to caribou meat, skins and bone tools.

CNN: mountain

Ridgeway set up a team of four men who would try to find and follow the migration route of the chiru more than 200 miles across the remote Chang Tang Plateau.

NPR: Chang Tang's Endangered Antelope

Another theory is that the birds may have fallen victim to hunting, development or changes in farming along their 6, 000km (3, 600 mile) migration route to breeding grounds in the Russian Arctic.

BBC: Red-breasted goose