I have owned my house for two years and every year I've had a return Bluebird pair, this year I finally put up a house! Within 30 minutes, there was a battle between my bluebirds and what I found out were tree swallows, so I ran out and got another box. Both have been nesting within 20 feet or so of each other quite peacefully since. On May 22, I found my first bluebird egg in the nest and today, six!! Out of the six of them, two have visible dents in them, and I haven't been able to find anything about them... I have had house sparrows around and I've installed fishing line onto the boxes. I've even gone to the extent of purchasing a pellet gun but so for they have eluded me. Any help I can get on if this is normal (which I don't believe is the case), or what I need to do to make sure the safety of my Bluebirds I would greatly appreciate it!!!