Kerfs on the outside sounds like a great idea. Have you seen her feet up close? We have had some blues with foot deformities on the forun before. Harold I think , had 2. I would go so far as to add a perch (under the Wren guard) if she is still having trouble. Make the Wren guard so it is at least 3" out from the box, or try the "training guard" first. I did just a short cardboard flap till my gal got the idea then did the full blown Wren guard. Add them togheter,less stress,they alreadty know the spooker!I am babysiting 18day old hide and peek babies at the moment! LOL

Rebecca, Good idea! It would be very durable and quick to install. My cardboard ones have sides, so if you figure out a way to do that with a tag I would be very interested in your result. With cardboard, I always put the "decorative" side toward the box so as not to possibly spook the birds. I think I would also do that with a tag and leave a 2.5-3 inch gap between the guard and the box. Please let me know how it turns out if you give it a try! I'm glad you started a new thread on this subject. The other one had gotten sooo absurd. That's partly why I resorted to Latin in that one. But Latin strains my brain and I am so glad to be typing in English again!

I have no idea when they would be "positively" safe but I'd wait at least 7 to 8 days with bluebirds and 12 to 13 will smaller species like Chickadees or Nuthatches. Linda had a thread here regarding a titmouse baby, 13 to 15 days old, still being harassed by HOWR entering the box. It was a single baby, the HOWR had already gotten rid of the others when they were younger, but the HOWR didn't seem to mind that it was close to fledging age and too large to physically remove. Thankfully that little TITU did fledge safely. In my last nesting of BHNU I left the wren guard on till they were 14 days old.

A lot of people ask me about wren guards, so I put up some information and photos about what they do and how to make one: http://www.pbase.com/gymell/wren_guard