In the Blink of Bird's Eye, a Model for Quantum Navigation

Wrt. strength change - the Earth's field would easily change 1% depending on your location, so definitely shouldn't be an issue for a sea/continent crossing birds.

What is special about the second field is that it was probably much more - several orders of magnitude - curved than the Earth's field (which for practical purposes of flying to Africa is a bunch of straight lines pointing to South). I'd bet those "ripples" and "criss-crosses" was what confused the bird. And in addition the field was oscillating :)

From the article :

"Moreover, in the 7-MHz oscillating field, this effect depended on the angle between the oscillating and the geomagnetic fields. The birds exhibited seasonally appropriate migratory orientation when the oscillating field was parallel to the geomagnetic field, but were disoriented when it was presented at a 24 or 48 degree angle. "