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The photo is of a Violet-green Swallow in a nest box with 5 Western Bluebird nestlings. The Violet-green Swallows have returned after migrating and are looking for cavities to nest in. The swallows won't harm the nestlings, and there are reports of swallows actually assisting in feeding the nestlings.

Violet-green Swallows and Ash-throated Flycatchers are the late nesters and often have to use whatever cavities are available. I've been finding both Tree and Violet-green Swallow nest starts in boxes from which other species have fledged and before I had a chance to clean the used nest out.

This is another example of the competition among birds that occurs for cavities to nest in. I added another nest box nearby in hopes that the swallows will use it.

Here's the swallow observation: https://www.inaturalist.org/observations/79034267

📸: By Lee Pauser

This observation was part of a nest box trail done in conjunction with the California Bluebird Recovery Program (CBRP), which runs the Cavity Nesters Recovery Program (CNRP). CNRP involves many volunteers across California who establish and monitor nest box trails for cavity-nesting birds. During the breeding season, these boxes are checked weekly for parasitism, predation, number of eggs, nestlings, and number of young fledged. Then at the end of the nesting season, volunteers submit their results to CBRP. We compile these results and submit them to the Cornell Lab of Ornithology’s NestWatch program and other interested organizations.