To prevent birds accessing your shop via the top of your roller door, get the missus to sew a pocket in a length of material that is wide enough to go from one side of the door to the other.
Go to your local junk yard, and rip the rubber door seals off old refrigerators and salvage the strip magnets from inside them.
Put these magnets into the pocket of the material, and attach the material to the top of the door frame with the magnets three or four inches down.
The slightest breeze will blow the magnets against the door where they will seal it, but not so tightly that the door becomes impossible to open.