We were at Hoffman Hills Recreation Area yesterday and spent about an hour watching several pair of Tree Swallows battle a pair of Bluebirds for a single bird house. Apparently there were multiple offers on the house and the contenders kept raising the offer.
The Bluebirds were the only ones that actually went into the house. The Tree Swallows sat on a branch about 20 feet away and would periodically make a run at the house. As soon as the swallows neared the house the Bluebirds would come back to the house and chase them away. The Swallows would the harass the Bluebirds. At the same time another pair of Tree Swallows were dive bombing the pair that was trying to get to the birdhouse.
More photos from Hoffman Hills can be found on my website.