New male bluebird in the yard with a nesting pair...

I know that I can count on everybody's advice.....

I have a pair of bluebirds that are using one of my nestboxes. Their babies are about 5-7 days old and all is well. Today, a different male showed up in the yard. At first, he flew around from tree to tree singing very softly and finally made his way over to the nestbox. My resident male took up a post on top of the pole where his box is mounted and only minded the new guy if he got within 10-15'. The female completely ignored him and went about her usual business of picking up the mealworms of sitting in the box with her head sticking out but the male looks very perturbed. This development completely blew my schedule out of the water as I had to hang out on the deck and watch to see what was going to happen. Is this new male a threat to their nestlings? Has anybody had this happen to them?