This is a picture of the sparrow, which is not a nice bird to the bluebird. The sparrow will often go into the home of a bluebird, or a swallow, and disrupt its nest, often times killing the eggs and the birds inside.
Sparrows are birds that you have to be very careful of when trying to restore the bluebird population. This bird will do everything it can to hinder your good intentions.

You know you have a sparrow in your bluebird box when the box is completely full with the nest, and often times litter is used in the nest.
Below is what Sparrow's eggs look like.