Anita, I have a similar situation in that my bluebirds are back for a second brood. The house wrens didn't bother the box during nest building (and the box had been empty for about a month and there were no sticks). Mama laid her 1st egg this morning and this evening, a house wren took out that egg and smashed it at the base of the box. I think that now I'd say "the rule is that there are NO RULES!" I've had house wrens attack several eggs or several chicks, never just one egg. As far as putting up a wren guard, the problem you have is that the house wren has been in the box. And where there is one house wren, there are usually more near by. The reasoning behind the general rule of putting the wren guard on the box when the 1st egg is laid is that the female is less likely to abandon the nest if she can't figure out the wren guard. It's also hard to say whether your female is 'with egg' and I'd hate to see her not be able to get into the box to lay her 1st egg, if that makes any sense. You could wait till it's afternoon and the bluebird hasn't laid an egg and put it on. If she can't figure it out, I'd take it off till the afternoon of the next day, etc. Good luck, Linda in NW GA...

Oh my, what a quandary! Our bluebirds have been battling HOWR too. But they have defended their territory and protected their 4 eggs. So far so good. The wren was removing nest material before the bluebird eggs were laid, so there was only a minimal amount of pine needles left in there. All the soft grasses had been removed by the #$@% wren. So the bluebird eggs are being incubated, but are nearly touching the wood floor. I've wondered if I should add some soft grasses, but then, I think, Mama knows best. Sometimes it is difficult to know what to do. Surely a wren guard would not help you at this point. Sorry, I'm no help at all. BlueBars

Boy can I relate to your frustration. Those wrens ruin so many nests. Sorry about your Blues being chased out. Those wrens are just awful.

I have seen tree swallows try to physically fight off those nasty house sparrows -- one Tree Swallow had a grip (in flight) of the HS. But the valiant attempt was not enough and the house sparrows prevailed until I intervened and ousted them. That was years ago and I'm not sure but IIRC the swallows already had a clutch of eggs. House wrens will often successfully butt in and pile nesting material right on top of a Tree Swallows nest. Wrens usually prevail. I recently read some blogs about house wrens -- not flattering. They can be persistent and harmful to other nesting birds. I will try to locate that link. Unlike house sparrows, wrens are native to USA and you cannot tamper with active nests. However their 'phony' nest piles of twigs, are fair game. Some people put an reversed "L" shaped structure at the top of the nest box that helps prevent the entry hole from being seen and dissuades wrens from entering with twigs. Tree swallows are not bullies, in my experience, but wrens and house sparrows sure are.