If you have wetlands, you most probably have snakes.
This year, we have had cardinals fighting with the bluebirds right at the bluebird house! I like the cage idea...we had 3 bluebird eggs last week, but try not to check too often so we don't draw attention. There is a hawk who seems to know when the babies will fledge, but fortunately, the blues do that before dawn, so far the blues seem to be increasing despite the hawk.