Migratory lids, or flat lids of any kind, may end up above top bars at  
the bee space distance or less depending on the design of frames and  
boxes.  That may lead to more binding than with inner covers spaced a  
bit further away from the top of frames.

Not exactly the same topic, but related: plastic frames tend to get  
propolized to lids or other frames above them much more than wooden  
frames.  Why? 

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html