The Magic (?) Stick

Ok, this is something I can’t explain – I’m hoping someone else can help.

The slideshow below consists of a series of images taken about an hour apart last June by one of the timelapse cameras at The Nature Conservancy’s Niobrara Valley Preserve.  Hover your mouse over the images and you can click the arrows to move between photos.  Watch the stick in the foreground…

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What in the world?  The stick doesn’t do it most days, but on some days, the timelapse camera captures one end of the stick rising into the air.  Overnight, it returns to its previous position.

Here it is again, a couple weeks later:

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Later in the season, the stick was dislodged so one end was no longer in the ground.  It still moved, but in more of a twisting motion.  I think the stick just to its left is moving slightly as well?

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This is not a living entity.  It’s a stick; dead and disconnected, but still able to move.

My best guess at an explanation is that the stick is moving because of moisture.  Over time, the stick absorbs moisture and then dries out, and the shrinking and swelling of the wood could change its shape.  Because humidity tends to rise overnight and fall during the day, that could explain the pattern.  But, if daily humidity patterns are affecting the stick, why doesn’t it happen more often?  Also, in the October series above, the stick was lifting as it rained, so that seems counter to my hypothesis…?

Anyone else have an explanation?  I love a good mystery…