What can I do to keep the birds (barn swallows I think) from building the mud nests under the eves--the birds do not bother me but the poop they drop on arrival makes a mess of the side of my shop.
This is an excellent time of year to address this issue. Your best strategy will be a 2-pronged approach.
First, remove the existing nests. Power-washers work really well for this, but long poles used to break up the mud also work. Be aware that various bird parasites over-winter in the nests - Humans are not the usual hosts for these pests, but it will be best if you can conduct the work in a way that will minimize your contact with the contents of the old nests, just to be on the better side.
Once the old nests are down, you need to exclude the birds from that area. One way to do that is to stretch (tight) netting (available in most home/bldg material stores) across the eaves area so that birds will encounter that before they can grab the side of the building to start construction of new nests. Realize that the birds are pretty adaptable in that if you only block access to one part of a favorite building's eaves, they might shift their nest-building to eaves down the way or around the corner.
The birds, their eggs, and their nestlings are federally protected. However as long as you do this nest-removal and blocking work prior to the next nesting season, you should have no troubles with avoiding potential harm to actual birds. If you're able to finish up your work on this by Feb/March, you should be well-prepared by the time the birds start migrating back into our area.