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...

For about 5 years there were a pair of Carolina Wrens who built a nest on our screen porch inside a grocery bag filled with other grocery bags that hung on a hook outside the kitchen door. The little rascals squeezed under the screen door through a space so small that until I saw one of them do it I would have said it was impossible. And each year when the nestlings fledged and left the nest there were always at least two of them who couldn't follow their parents under the door. My wife or I would have to leave the door hooked open so they could leave. Once it took three days for one chick to figure it out, even with its parents repeatedly coming in to lead it from captivity.

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.

Cute, tmc. Do you know how many are in there? Do you have that box mounted on a pole?