I believe that you are suppose to put a wren guard on after the first egg is laid. Check out the link below for more information.

Here is a link that might be useful: info on wren guards

Renee, Thanks for the info. I should have mentioned I had read that and I'm still a little uncertain. I found both following statments at that website....

"Bluebirds tend to be more tolerant of box changes, and it may be wise to put up the wren guard as soon as the bluebirds start a nest, since once the House Wren finds the box, the guard is less effective at deterring them."

"Put up the guard after the first egg (or two or three) of the desired-cavity nester has been laid. (On occasion, people have put the guard up before an egg was laid, and have had it accepted, especially if the parents are tolerant of monitoring."

I know these blues are pretty tolerant, but I just ripped out the nest they were working on (to remove the old nest with unviable eggs). I'm afraid if I put the guard on now, they may have just had enough disturbances to give up. I'm also afraid to wait too long, and have the wrens discover the box, because I'm pretty sure once that happens, the wren guard won't be much help. I was hoping someone could share some personal success that could help me decide how long to wait.

Thanks again Renee.

Wren guards are a relatively new idea, as are sparrow spookers. On Bet's web page, I'm sure she is gathering information from the experiences of others as well as her own observations. Everyone's situation could be a bit different. There are no rules that cannot be broken by wrens or house sparrows. We just have to take our best guesses, and feedback to others what we learn as we go.
Anyway, to help with your question, if you are not sure about the bluebirds' commitment to build, make sure the bluebirds are still active. Placing a long piece of grass in the entrance hole can be helpful to determine that someone has been in or out. Check to see if more nesting material has been added--if you are not sure, comparing digital photos each day can help. Other than that, you will have to take your best guess. Be sure to report back with your observations, as it may help someone else.
Also, take notes on a calendar. You might be surprised how much those notes will help you later.
BlueBars

Bluebars, you are so right.

"There are no rules that cannot be broken by wrens or house sparrows."

And in my few experiences, house wrens are tougher to deal with than HOSP. Many THANKS to Bet and others who provide such helpful information. Her website is, as my teenage son would say, "stinkin' sweet". [That's a good thing ;)]

The blues did stay near, in, or on the box most of the day after I removed the nests. They did a little more nest building, though not as much as I would have hoped for. Right now I'm feeling like my "best guess" would be to let them do a little more nest building in the morning and maybe put the wren nest on in the early afternoon to give them plenty of time to accept it. I have to admit I'm nervous that they may not be attached enough without eggs. And I'm also nervous about waiting because the HOWR are SO CLOSE. Whatever I do, I'll let you all know how it goes. Thanks for the moral support.

Beverly

Bev I use guards every year and I've never tried using one before first egg so I'd be interested to see if it works. I know Dees will accept one before first egg but I don't know about blues. I have a lot of wrens and I always use a guard with sides on it. I think it hides the entrance better.
Cathy