Oops! I forgot to mention that a) this area is where I keep my compost; and b) some of the holes seem to have a "guard bee" inside - bees will attempt to enter into a ground hole (with or without the surrounding mound) only to back out. If you look in the hole, you can just make out the head of a bee sitting less than a half-inch inside the hole. They don't move, or appear to react to the ones attempting to enter, but they just sit there.

This is not swarming, nor are they honey bees. They are ground nesting bee and what you are seeing is just their normal day to day activity and the holes are their nests. Depending on the species ground bees may have only 1 per nest to several hundred (far fewer than a honey bee colony at 10 to 60 thousand). I'm not terribly familiar with them so I can't really identify what species you have from the description.

Here is a link that might be useful: Ground Nesting Bees and Wasps

Probably it is a harmless wild bee?...there are about 200 species.

Can you post a pic?
Konrad

Here's a link to some close-up shots of the bees and their burrow entrances. Unfortunately, I couldn't get a good shot of the entire courtyard covered w/bees. But to give you an idea, the area is about 15x7 feet, and I have to literally watch every step to make sure I don't step on a (or a few) bees.

Here is a link that might be useful: Tsepina's photo portfolio

I'd guess (I'm no expert) from the pictures that they might be "Plasterer" or "Polyester" bees. Though they are fairly close in appearance to several others. See the pictures on the following link:

Here is a link that might be useful: Bee Photo Gallery

Nice Pics!
This is something I thought you have.....they like sandy soil and are harmless, some kind of wild mason bees...not to worry about it, they never swarm like honey bees do.
Konrad

Thanks (for both info and compliment)! - I wish I could get some closer shots, but the camera's focus gets fuzzy when I try :P.