The longitude problem (determining East-West position) is a classical problem in human sea navigation. Prior to the use of GPS satellites, extraordinarily accurate clocks measuring the difference between local time and a fixed reference (e.g., GMT) were needed to determine longitude. Birds do not appear to possess a time-difference clock sense. Nevertheless, experienced night-migratory songbirds can correct for East-West displacements to unknown locations. Consequently, migratory birds must solve the longitude problem in a different way, but how they do so has remained a scientific mystery. Here we suggest that experienced adult Eurasian reed warblers (Acrocephalus scirpaceus) can use magnetic declination (the difference in direction between geographic and magnetic North) to solve the longitude-problem at least under some circumstances under clear skies. Experienced migrants tested during autumn migration in Rybachy, Russia were exposed to an 8.5° change in declination while all other cues remained unchanged. This corresponds to a virtual magnetic displacement to Scotland if and only if magnetic declination is a part of their map. The adult migrants responded by changing their heading by 151° from WSW to ESE, consistent with compensation for the virtual magnetic displacement. Juvenile migrants that had not yet established a navigational map also oriented WSW at the capture site, but became randomly oriented when the magnetic declination was shifted 8.5°. In combination with latitudinal cues, which birds are known to detect and use, magnetic declination could provide the mostly east-west component for a true bi-coordinate navigation system under clear skies for experienced migratory birds in some areas of the globe.