This happened at my house in my T-111 siding, behind the window shudders. Here's a few pictures of them that I took when I had the shudders off for painting.
http://www.homeorchardsociety.org/article/23/

The bees are cocooned-in and dormant right now. If you caulk them in they might be unable to get out when they hatch in April of next year. That would be fatal.
Can you get at the cocoons? Sliding a small screw-driver underneath them is useful for lifting them out. If you can carefully remove the little cocoons (see picture #3 in the link above) then you can just put them in a little box or bag and leave them in a *dry* place outside. They'll hatch in April and merrily fly back to your cedar siding to nest again. Unless you've caulked the openings, in which case, you'll be OK.

Check that, the larval mason bees are probably not in a cocoon stage yet. Moving them now would most definitely be fatal. If you can wait til September, they should be in cocoons and ready for *careful* relocation.

I had orchard mason bees in my T-111 siding for years and they never caused any problems. Every spring they'd hatch and pollinate like crazy. There was never any mess, stains, bees inside the house or anything noticeable. I have since moved and I wish I had them in my vinyl siding.