Thanks for the tips, you blokes
That story of the broken-legged climbing fox...
We built the main coop 15 years back, with a chicken wire floor, to prevent tunneling attack.
All the walls and roof are either corrugated iron or chicken wire.
We put solid roofs on because of eagles and hawks in the area, and we'd heard that foxes can climb trees; they're more akin to cats than dogs.
When the "mother" was taken, I'd left the door open after letting the rest of the adults out for the day.
They're free-range until they go to roost, then they're locked in again.
The chicks were in a second shed of wood, wire & corrugated iron.
Hens usually attack chicks that they haven't hatched themselves, even though it's their eggs.
It wasn't safe to put the chicks in with the others without a "mother" for protection.
Our ground is usually that hard, it's an effort to get a spade into it, but all the same, we set house bricks along the door side.
The foxes ripped some wood then dug between the wood and bricks until a couple dropped into the hole. From there on, they just went deeper until they could slip under the wood.
The ground inside was much softer, as guinea pigs lived there for some years, so there was a lot of broken down straw, wood shavings etc. Extending the hole inside would have been easy going.
If we need to use that shed again, be assured, there'll be concrete reinforcing mesh similar to Joe's diagram.
