NEST-SITE SELECTION IN A HIGH-DENSITY COLONY OF BURROWING OWLS
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2018 The Raptor Research Foundation, Inc. Although many Western Burrowing Owl (Athene cunicularia hypugaea) populations have been declining, a high-density population nests at the National Radio Transmission Facility (NRTF), Dixon, California. We compared density at NRTF to densities elsewhere, and nest-site use and reuse to available nest substrates. Breeding pairs numbered 24-44 per year, averaging 34 pairs on 83 ha, the fourth-highest on record. Occupancy of eight artificial nest sites installed in 2000 declined from six pairs in 2006 to one pair in 2007 and 2008, and none afterward. Nearest-neighbor distance among artificial nests averaged half the distance among nest sites in fossorial mammal burrows and concrete half-rounds covering aboveground power cables. Undisturbed clay soils supported pocket gophers (Thomomys bottae) but few ground squirrels (Spermophilus beecheyi), whereas disturbed soils supported both ground squirrels and Burrowing Owls. The presence of both ground squirrels and Burrowing Owls was associated with backfill soils over buried cable, cable covers, and areas where soils bordered impervious surfaces. Nest-site reuse was low, with only 12% of the sites occupied in all study years, 2006-2011. Most (78%) nest sites reused in a subsequent year involved nests in a different burrow or cable cover opening >1 m from the previous year's nest. We recommend research on whether concrete half-rounds might outperform buried utility boxes as artificial nests, especially in conjunction with efforts to conserve the fossorial mammals that naturally excavate burrows used by Burrowing Owls.
