The last time I raised chickens I lost a bunch of them – to critters that dug under the fenced-in chicken run I'd built. I didn't know – then – about the importance of burying the fence several inches, in order to thwart the critters looking for a chicken dinner. I know better now. And if you're reading this before you get chickens . . . now you do, too.
My run consists of a three-sided square of fencing and posts – the fourth side being the back wall of my shed, onto which I have built the coop. The posts are set about four feet apart and four feet deep. In between each post is a trench about six inches deep. Into this trench I set the fence – which is made of fairly thick-gauge steel wire; you can buy this stuff in rolls at places like Tractor Supply, Rural King and so on. Then I poured a bead of concrete into the trench.
This anchors the fence into the ground and makes it very hard for critters to get under and into the yard. I also poured a pad of concrete under the swing fence entrance to the coop, leaving less than 2 inches of gap between the bottom of the swing fence and the concrete pad – also done to make it difficult for a critter to have dinner.
My fence will protect the birds from stray dogs, coyotes and foxes. Only a bear or a two-legged varmint of another sort will be able to get through it. And I may run a hot wire along the top of it, to prevent anything capable of it from crawling over it.