Hi Adrian
I have also noticed differences according to the speed of flight. I had a
row of hives at one time along a post and rail fence, but about six feet
from it. Along the top of the fence was a run of chicken wire with nominal
19-25 mm holes.
The hive entrances were facing the fence and bees on the leaving leg of
their flight, might fly through the gaps in the fence or through the mesh,
but bees on the return journey went mainly over the top edge of the wire,
very rarely would they fly through the mesh, and I do not remember seeing
any returning bees fly through the fence itself.
Best Regards & 73s, Dave Cushman
Beekeeping and Bee Breeding Website
http://website.lineone.net/~dave.cushman