If I tried to tarp their run, any wind would scare them senseless and any rain would cause the entire thing to collapse.
My girls don't free range because I don't want to accept the risk. We have a hawk's nest on our property and my house cameras have caught fox and coyote at night; I've also seen a red fox roll up during the day.
The past two years have been a great reminder that the risk profile matters. If every wild bird carried the deadly flu, my flock would be locked in my barn. But - according to the data - the chances of them getting anything from a stray bird turd are slim to none, so I'm not going to lock them inside and make them miserable for months on end.
And also, the risk of them killing each other while locked in a space that's wing-to-wing 24/7 is greater than the bird flu. So, they'll keep living their happy chicken lives, unaware that the sky is falling. And if it does, I'll play the lottery, because those would be some serious odds.