Burrowing Owls Require Mutualist Species and Ample Interior Habitat Space

Mitigating habitat loss of western burrowing owls (Athene cunicularia hypugaea) often involves relocation from California ground squirrel (Otospermophilus beecheyi) burrows to offsite nest boxes. Naval Air Station Lemoore (NASL), Kings and Fresno counties, California, initiated this approach to displace a regionally important population from airfield grasslands. We examined monitoring data of burrowing owls and fossorial mammals at NASL to assess mitigation options. Occupied nests increased by 33 (61%), with 47 nest box installations in 1997–2001, peaked at 87 in 1999, then declined by 50 through 2013. Although ≥13 nest boxes were occupied in 2000, none were occupied in 2003–2013. Within a 43.1 ha isolated grassland monitored for 13 years, nest site reuse in ground squirrel burrows averaged only 17% between any 2 consecutive years. Compared to the average density across grassland study areas, ground squirrel burrow systems/ha numbered 43% higher within 60 m of occupied nests and non-breeding-season burrows. Vegetation clearing to restore kangaroo rat (Dipodomys n. nitratoides) habitat preceded a 7.4-fold increase in ground squirrel burrow systems and a 4-fold increase in occupied nests, but drought-induced extirpation of ground squirrels eliminated occupied nests from the 43.1 ha grassland study area. Ground cover near occupied nests averaged 58% of the mean vegetation height and 67% of the mean percentage of bare ground in the field. Both nest sites and non-breeding-season burrows occurred >60 m interior to field edges 1.4 times more than expected. Non-breeding-season burrows averaged 328 m from same-year nest sites, and only 7% of non-breeding-season burrows were also used as nest sites. Mitigating habitat loss should be made more effective by fostering natural burrow construction by fossorial mammals on patches of short-stature vegetation that is sufficiently expansive to support breeding colonies of ≥12 pairs averaging ≥60 m from the field’s edge and a separation between non-breeding-season burrows and nest burrows minimally equal to mean nearest-neighbor distances among nests.