We did that for the swallows until it rained RtB, but our soil is so free draining that it was impossible to keep the soil wet, especially as we rely on stored rainwater and a well, with no mains, so not much water to spare. I assume the martins pop down to the stream which is a field away, or the burn which is about a mile away. There seem to be 5 martins all around one nest - there is another nest box, but they are ignoring it. They have knocked off most of the mud they added today, so we need to have another think of how to make mud stick! Any ideas?