Migration?

[Rebecca Jolly]

     I was wondering what drives butterflies and moths to 
     migrate (in this case northwards into southern UK), 
     into areas in which they cannot survive the winter.  Do 
     some of the later/autumn generation migrate south to 
     redress the balance?
     
     I guess it is an attempt to colonise new areas and 
     exploit new regions where there may be more food 
     available or less disease/predation.
     I would be interested to 'hear' your thoughts.
     
     Best wishes
     Rebecca Jolly