Defenders' Experts
Background
The western burrowing owl is a small ground-nesting bird of prairie and grassland habitats. The western burrowing owl has declined significantly throughout its range in North America and is listed as endangered in Canada in which it breeds and threatened in Mexico.
The majority of the mid-western and western states within the owl’s range have recognized that western burrowing owls are in trouble: it is state-listed as endangered in Minnesota and Iowa, threatened in Colorado, and as a state Species of Special Concern in Kansas, Nebraska, Oklahoma, South Dakota, North Dakota, Montana, Idaho, Utah, Washington, Oregon, and California. California supports the largest remaining breeding and wintering populations of the species.
Burrowing Owls in California
Burrowing owls in California rely upon burrows dug by ground squirrels for nests, and require suitable habitat consisting of open fields with adequate food supply for foraging, low vegetative cover to allow owls to watch for predators, and roosting sites.
Many early accounts of the burrowing owl reported the species was one of the most common birds in California. Burrowing owls historically ranged throughout the Central Valley, were found in suitable habitat in coastal areas from Marin County south to the Mexican border, and in sparsely inhabited desert areas in northeastern and southeastern California.
Declining Populations Documented
Since at least the 1940s, owls have been in continuous decline throughout the state. Surveys conducted throughout most of California during the early 1990s documented a nearly 60% loss in the number of breeding owl colonies known from the 1980s, and a decline in overall population numbers by 8% per year. It was thought that an estimated 9,450 breeding pairs of owls remained statewide at that time.
Breeding owls have recently been completely eliminated from 5 counties (Napa, Marin, San Francisco, Santa Cruz, and Ventura) and are nearing extirpation in at least 6 others (Sonoma, San Mateo, Monterey, San Luis Obispo, Santa Barbara, and Orange). Breeding owls are rapidly disappearing from southern Los Angeles, western San Bernardino, western Riverside, and San Diego Counties as well.
Threats
Burrowing owls are threatened primarily by habitat loss to urban development and eradication of ground squirrels and other burrowing rodents. The state-approved practice of relocation of owls from development sites is accelerating local extirpations from rapidly urbanizing areas.
Burrowing Owls, Agriculture and Conversion of Agricultural Lands
While burrowing owl populations are declining in urban areas, densities of owls in some areas of the state have increased with intensive agriculture, such as in the Imperial Valley, southern Central Valley, and lower Colorado River Valley. Over 71% of California’s breeding owls currently live in the margins of agricultural land in the Imperial Valley, an area that comprises only 2.5% percent of the land area of the state.
Owls in the Imperial Valley, which primarily nest in burrows in earthen irrigation channels, are facing threats from conversion of agricultural lands to urban development, plans to line earthen canals with concrete, and ground squirrel eradication programs.
Over 15% of the state’s breeding owls reside in the southern Central Valley, an area undergoing explosive human population growth and rapid conversion of agricultural lands to urban development.
Additional Threats
Other factors contributing to the decline of owls statewide include destruction of burrows through disking and grading, impacts of pesticides, increased predation by non-native or feral species, habitat fragmentation, and other human-caused mortality from vehicle strikes, electrified fences, collisions with wind turbines, shooting, and vandalism of nesting sites.
Protection
There are currently no state or federal laws that protect owl habitat and such habitat is rarely purchased by agencies to conserve the owl and other grassland-dependent species. An estimated 91% of all owls remaining in California occur on private land, much of which is threatened by future development.
Although federally designated as a Species of Special Concern in 1994, federal regulatory mechanisms such as Habitat Conservation Plans have proved inadequate in protecting significant owl habitat or stopping the rapid decline of the species.
State regulatory mechanisms, such as designation as a state Species of Special Concern in 1979, adoption of burrowing owl mitigation guidelines by the California Department of Fish and Game in 1995, state Fish and Game Codes protecting nesting raptors, and limited creation of mitigation banks to purchase habitat, have also proved unsuccessful in protecting the burrowing owl and its habitat.
Throughout the vast majority of the burrowing owl’s range in California, breeding owls persist in only small, declining populations of birds that are highly susceptible to extirpation. The burrowing owl is in imminent danger of becoming extinct throughout a significant portion of its range in California, and requires immediate protection as an endangered or threatened species.
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