Western burrowing owls (Athene cunicularia hypugaea) are year-round residents at low elevation (0-18 m) breeding sites in Santa Clara County, California, U.S.A. Burrowing owls have also been anecdotally documented during winter in this region at non-breeding sites, but the relationship between burrowing owls seen in winter and resident birds has not been previously determined. The objective of this study was to assess the spatial and temporal associations between resident burrowing owls and burrowing owls observed in winter. Burrowing owls had been captured and banded at known breeding sites for a number of summers before the study, and summer banding continued during this study. In addition, during four winters, 2014-2018, we banded burrowing owls at the known breeding sites and at other locations around the County where burrowing owls were seen by us or other birders.
Results/Conclusions
During four years of study in Santa Clara County, we found burrowing owls each winter at high elevation sites (approximately 80-260 m), where birds were not known to breed, and at the low elevation breeding sites. We never resighted a banded resident bird at a high-elevation winter site. Similarly, no birds banded in winter at high elevation were resighted during the breeding season either at any of the breeding sites or at the sites where they wintered. In addition to owls wintering at high elevation sites, other owls joined resident birds at the low elevation breeding sites each winter and then disappeared before the next breeding season. Eight burrowing owls we banded in one winter were seen the next winter, but not in the intervening summer. These findings show burrowing owls in this region exhibit partial migration, in which owls from other parts of the range winter with resident birds--as well as wintering in other areas of the region--but then leave to breed elsewhere. This migration pattern may have implications for the persistence of this and other resident burrowing owl populations.