In reply to Rog Wilko:
.. or drives them back South for a few hundred km.
Last year I attended a guest lecture at my uni about tracking the migration of individual, transmitter tagged songbirds along the Dutch/German/Danish North sea coast using an array of antennas normally used for air traffic control.
The surprising finding was that even in normal weather the migration is anything but linear, but individual birds would migrate two hops forward and one back, sometimes moving out to Helgoland or the North Frisian Islands before dropping back across the sea to the Dutch mainland.
In inclement weather, in particular rain and headwind, many birds would allow themselves to drift backwards in small hops even out of range of the tracking area, presumably to France.
CB