A pest-control expert on TV recently said (of mice getting into houses) if
a
biro would go through a gap, a mouse could.
I've seen young mice feeding with mum on our bird feeders and can accept
that.

He also mentioned the skull thing.

The size of mice skulls is used to advantage by beekeepers. In winter mice
like to go into hives - sheltered, warm and food immediately available. We
tack a strip of metal over the long, narrow hive entrance, the metal is
pierced with 3/8" diameter holes. Honey bees can get in and out easily. The
skull's vertical measurement is less than 3/8" but its width is more, so it
can't get in.

It always works.

I don't know what the skull's vertical measurement is but mice can get
through the tiny gap between skirting boards and floorboard in our house.

Mary