Worm Holes

The trails in the woods behind our house are pretty much down to bare dirt from dragging trees & shrubs back to the brush pile. I've noticed a lot of little piles of twigs on the trails. They almost look like little tufts of dormant grass growing out of the soil. I pushed one aside with my foot and noticed that it consisted of sticks and twigs pushed in to a small hole about a half-inch or less in diameter.

I took some pictures today, in the order shown here. I pushed some of the piles aside and exposed various worms in some of those holes and in some of the overturned piles. The holes that look bigger are just close-ups of the previous one. I don't know whether these animals are at all responsible for making the holes or the piles of sticks, or whether they're just taking advantage of the situation. I tried to look up this phenomenon on the internet, but haven't found a definitive cause yet.




One interesting discussion was at http://blogs.exploratorium.edu/whyintercept/2011/03/30/earth-worminess/, but it didn't really explain what kind of worms do this, nor how.

It's hard to imagine an earth worm pulling sticks down into its hole. Since it seems like I've only noticed these piles after hard rains, I thought that maybe the worms burrow up during the rain, and the sticks get washed into the hole by the water flowing down the trail. But I can see the holes in that argument too.

Update, May 20, 20011:

• This article on no-till farming explains that earthworms actually do pull sticks down into their holes.
• Great Lakes Worm Watch explains: "Nightcrawler middens, are distinctive piles of cast material around the openings to their burrows. These middens are usually about 1-5cm in diameter and 1-3cm in height with a burrow hole (2-4mm in diameter) near the center. The burrow entrances of middens also often have large numbers of leaf petioles or fragments of leaves sticking out of them. These got stuck there as the nightcrawler inhabitants attempt to pull leaves down into their burrows."
• This is a good place to insert the fact that earthworms are not native to this region, at least they weren't here for the period between the last ice age and European settlement of the area. There's evidence that although earthworms are beneficial for growing farm crops, they are detrimental to woodlands.