However, previous research had suggested a much more confusing picture.
Migratory birds are born with an innate magnetic compass preference that coincides with their species' migratory direction. Previous research suggested that before the migration period, songbirds are able to recalibrate the magnetic compass when exposed to a "conflict" between magnetic and celestial (including polarized light) cues, but during migration it appeared that the reverse was true, -- the magnetic field was used as the primary reference for calibrating the birds' other compass systems. But in a few experiments with birds during migration, the birds did recalibrate the magnetic compass.
When Muheim and Phillips did a literature review, they noticed a difference between the experiments of the few scientists who saw migratory birds recalibrate their compass and of those whose birds failed to recalibrate.
"It is important how you do the experiments. It turns out that the part of the sky that matters is just above the horizon," said Phillips. "In cue conflict experiments carried out before migration, birds were usually housed in outdoor aviaries in a rotated magnetic field, where they had a view of the whole sky, including the horizon. Once migration starts, however, scientists usually exposed birds in "funnel cages". This is so, after exposure to the cue conflict, the birds' directional preferences could be recorded; songbirds in migratory condition leave tracks or scratches on the sides of the funnel as they attempt to take flight in the migratory direction. A problem arises, however, because funnels block the lower 20 degrees of the sky. In the only two experiments (out of 30 or so) carried out during migration where bi