Tree swallows are the water surface skimmers/acrobats.
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Tree swallows are often confused for PM's. They tricked me for a few months, too. They are much more welcome than the junk birds; house sparrows, starlings, grackles.
We will put 2 blue bird houses fairly close together; maybe 75 feet apart. Tree swallows will typically establish one house pretty quickly, then defend and ward off all other tree swallows. Then the blue birds will establish at the other house. The tree swallows in the first house won't care about BB neighbors, and will help keep other birds away from both houses.
