squirrels love to dig in my containers whenever the can. usually in the early morning before I get out there.
I think they to bury their nuts in the soft soil rather than the hard ground. It really got aggravating and yes birds poop on them too.
only solution I have come up with is to get a bag of small to medium size rocks from home depot or lowe's and cover all exposed dirt.
like these to help deter squirrels from digging.