Last year my son built a birdhouse in his first year of woodworking in junior high. I was so proud I painted a cute garden scene on the front and we put it on a fencepost in the backyard.
Before long I identified the male wren bringing sticks to the house and singing happily. We were all excited when he called in a mate and she began to bring grasses and fluff to the house. Before long we saw the male bringing her food while she sat.
The wrens didn't seem to mind us in the yard even sitting in chairs nearby. Before long though, a sparrow began sitting on the perch and blocking the male wren from feeding the female.
I tried repeatedly to drive away the sparrow and even resorted to "guarding" the house every time I was out by spraying the sparrow with the hose. Unfortunately we stopped seeing the wrens and later discovered all their eggs unhatched in the nest.
This year when nesting began the wrens returned and as soon as the male began bringing sticks the same male sparrow returned and sat blocking the nest.
We removed the perch after some research and thought that was the end of the problem but the male sparrow began sitting in the tree and wouldn't leave even though we saw and heard the male wren vigorously trying to drive him away.
My husband commented that he saw quite a squabble going on on the ground near the birdhouse but we assumed it was mating or fighting sparrows.
Later that day we were shocked and saddened to find the house wren on the ground dead under his house.
While we had originally read that the hole should be no more than 1 and a quarter wide we now have found it can be 1 and an eighth and are trying a board with a new smaller hole over the opening because the sparrow was able to get in the 1 and a fourth and we saw him coming and going from the plundered house.
With the new smaller opening the sparrow can't get in even though he started trying as soon as my husband finished nailing it on.
Hopefully this will make him go away, otherwise we may take harsher measures since the sparrow is a nuisance and now an obvious killer of the natural species.
I didn't know prior to this that they were so nasty, but have learned from reading online that sparrows are to be discouraged and are an invasive destructive species that not only take the food supply of other birds but also kill and take over nesting sights.