What we were taught in behavioural biology is that some of the larger birds that migrate can 'navigate' through various means including:
Using the sun/ stars
Sensing earths magnetic fields
Accurate time keeping
Orientation info from semi-circular canals
I don't see why the larger birds couldn't fly through clouds..
The smaller birds that 'pilot', not navigate might have a problem though, as they use visual clues for direction finding. I might crack open the textbook if I can find it..
Tom.
Edited to make sense..
[ 16 July 2001: Message edited by: tom775257 ]