I've received some comments and questions about the House Wren's behavior in the destruction of the Chickadee eggs. One of the best websites I've found to discuss it is this one, which presents the issues that cavity nesting birds face concerning House Wrens.
This website has some pretty amazing documentation on it, including a video of a House Wren removing young chicks from an Eastern Bluebird House, twice!! They survived, thanks to monitoring of the box by the owners.
And some more history:
House Wrens and Carolina Chickadees are both native birds and thus protected under the Migratory Bird Treaty Act. There are ways to deter the wren from nesting without harming it. So, I acted in that vein by removing the nest box before it could become an active nest for the wren. (Females arrive about 9 days after the males.) Chances are that the wren would never have allowed the chickadees back in without more harm. The chickadees are still together, exhibiting pair-bonding behavior and my hope is that they have found a cavity in a tree to try nesting again. Third time's a charm???