Bumblebees frequently investigate birdhouses with active bird nests.  While bumblebees can take over a bird nest for use as their own nest, the birds at The Retreat have won the day so far.

In this video, two different bumblebee invasions of a Titmouse nest are shown.  The first invasion was of a nest under construction (no eggs or young).  The female Titmouse exited immediately upon encountering the bee.  In the second invasion, it was a very different story.  The Titmouse nest was full of eggs.  When the female Titmouse returned to the nest, she clearly heard the bee.  Instead of fleeing, she went on a mission to hunt down the intruder.