Bird tunnels

I'm not sure how these curious little tunnels form. I don't know if birds actually make them. But they sure know how to put them to good use.
We've seen wrens hopping along the paths, jumping up every now and then to grab a bug from the underside of the lip.

These are along the edge of an old logging road on our property. I've only seen them at the higher altitudes, where the ground is rockier.
If you know any more about how they're made, I'd love to hear it.
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I'd planned that this post would describe a local political rally from the weekend, when we had not one, but two, presidential contenders in Birmingham. I'd have shown you lots of pictures, and related every little thrill. But I got sick, and can only relate how thrilling it was to watch it all on tv.
I have to say thanks to WBRC6, our local Fox channel, for showing both live. You can see the raw video of either appearance (Huckabee or Obama) by clicking on that link.
Too bad they don't show that huge honking anchor desk that they dragged all the way down to Bartow Arena for the reporters to sit behind.