Migrants and the problem of longitude

How do migratory birds like the Eurasian reed warbler (Acrocephalus scirpaceus) know which way to go when heading south for the winter? Indeed, how do they find their way back from sub-Saharan Africa in spring to breed among the reeds in the wetlands of Europe and Asia?
The answer may lie with magnetic lines and how they change depending on where on the globe you are.
“It seems that…the reed warbler may have a geographic map or memory that enables it to identify its longitudinal position on the globe, only by detecting the magnetic north pole and its variance from true north,” to quote one expert in the article. The birds can detect magnetic declination in other words.