Many birds don't sense smell in a useful way, the ones that do can sense smell really -really well.
It has to do with conservation of grey matter, all they need programmed in as instinct + whatever they can learn.
All birds seem to have fairly able ears. But if you hear something you can't see...
And what about weasels? Can't they get through a 2x4" hole? - yes and snakes and rats...
I have 1/2' hardware cloth all over- it is fine. I have runs of 12 feet at a time w/o cross support.
The pitbull and greyhound have bounced off at full speed (well full pitbull speed, half speed...