On Tuesday at Afton State Park, Erika and I saw something strange at a bluebird house. The opening of the box was protected from mammalian predators by a wire sleeve. We discovered a House Sparrow huddled in the sleeve.
House Sparrows, Tree Swallows, and House Wrens all compete for use of Eastern Bluebird houses. These species will even kill bluebirds during territorial fighting. Even competing female bluebirds are known to kill bluebird competitors. Cold and/or wet weather, however, may be the leading cause of bluebird fatalities (Gowaty and Plissner 2015).