High First-Year Occupancy and Success by Cavity-Nesting Birds in Nest Boxes at a Woodland Mitigation Site near Sacramento, California

We installed 19 songbird nest boxes prior to the 2024 nesting season at a woodland mitigation site in Rossmoor Bar Park within the American River Parkway in Sacramento County, California. We monitored occupancy, incidence of double-brooding, and reproductive success (nest success and productivity). In 2024, all boxes were occupied, with Tree Swallows (Tachycineta bicolor) by far the most common species (18 pairs, 25 nesting attempts). Two Oak Titmouse (Baeolophus inornatus) pairs and two Ash-throated Flycatcher pairs (Myiarchus cinerascens) each nested once in boxes. Ninety-two percent of Tree Swallow nesting attempts were successful, fledging an average of 4.4 young per nesting attempt and 5.8 young per nesting pair. Occupancy and reproductive success at Rossmoor Bar were high compared to other studies, indicating that the area is high quality habitat, especially for nesting Tree Swallows, nest sites appear to be limiting to population, and an abundant source of colonists is available to occupy boxes. Our results suggest that nest boxes should be incorporated into woodland mitigation programs to immediately compensate for losses of nesting habitat that will not become available again for decades in planted mitigation areas.