I heard a House Wren in this area on 5/11 and installed a wren guard. On 5/14/08, I found the entire clutch of bluebird eggs on the ground below the nestbox. The developing embryos are visible inside the broken eggs. These eggs were within a day or two of hatching.
The box already had a sparrow spooker on it, so I do not suspect House Sparrows, as spookers are more effective at preventing House Sparrow attacks than wren guards are at deterring House Wrens. (Sparrow spookers do not deter House Wrens.)
See related video showing a House Wren removing one-day old bluebird nestlings from a box.
House Wrens are the biggest problem on my trail other than paper wasps. (House Sparrows are regularly controlled via passive and active methods and no longer pose a serious problem.)
You cannot begin to preserve any species of animal unless you preserve the habitat in which it dwells. Disturb or destroy that habitat and you will exterminate the species as surely as if you had shot it. So conservation means that you have to preserve forest and grassland, river and lake, even the sea itself. This is vital not only for the preservation of animal life generally, but for the future existence of man himself—a point that seems to escape many people.
-Gerald Durrell, The Nature Conservancy
