Important Bird Areas

South Fork Kern River Valley

California

The Kern River is one of the major rivers of the Sierra Nevada, and its watershed, which extends from the highest point in the state south and west into the southern San Joaquin Valley, is totally isolated from other major drainages of the Sierra and the Central Valley. Its two main arms, the North Fork, which drains the High Sierra, and the South Fork, which originates on the Kern Plateau (the southern terminus of the Sierra), meet in a large reservoir called Lake Isabella, which straddles five major bioregions: the Great Basin, the Mojave Desert, the Sierra Nevada, the Central Valley and Coastal California. The South Fork Kern River Valley, about 15 miles in length, contains elements of all of these ecological zones, as well as one of the largest and best-preserved examples of lowland riparian woodland (Fremont Cottonwood-willow) in the state. Other major habitat communities include Joshua Tree woodland, wet meadow, freshwater marsh, Mojave Desert scrub, desert chaparral, and annual grassland. Though much of the 10,000-acre valley floor is privately-held by large cattle ranches, several thousand acres are protected as conservation lands from the eastern edge of Isabella Reservoir east past the community of Onyx, including South Fork Wildlife Area (1200 acres, USFS) at the eastern edge of Lake Isabella; the Kern River Preserve (1100 acres, National Audubon Society) just to the east; and Canebrake Ecological Reserve (1300 acres, DFG) beyond Onyx.

For more information: http://www.audubon.org/local/sanctuary/kernriver/

{link:For IBA map, click here.|http://www.audubon.org/bird/iba/maps/CA/CA217m_South_Fork_Kern_River_Val...}

Ornithological Summary

This IBA is best known for supporting one of a handful of large populations of Yellow-billed Cuckoo left in the western U.S. (ave. 40 birds/summer). It is the metropolis of the Kern Red-winged Blackbird, a poorly-known race confined to the lower Kern River watershed, and supports California's largest population of Summer Tanager, with an estimated 80 birds summering (S. Laymon, in litt.). Its breeding Willow Flycatchers (20-30 pr. each year) is one of the largest population of the federally threatened Southwestern race in the world. Between 11 and 16 pairs of Brown-crested Flycatchers breed here, the state's largest aggregations away from the Lower Colorado River. The riparian bird community is exceptionally rich, with 95 species documented as nesters, and over 130 species breeding in the valley as a whole. Mid-summer bird censuses have documented around 300 Yellow Warblers (most singing males) and over 1000 Song Sparrows covering only about 50% of the habitat (BB). The wetlands in the IBA support large numbers of nesting Tricolored Blackbirds (up to 1500 pr., BB), which feed with other blackbirds in the agricultural fields and feedlots in the valley. Alkali meadows and wet grasslands here support an interesting interior-coastal blend of species, including White-tailed Kite, Northern Harrier, Common Snipe and Grasshopper Sparrow. Spring migration can be spectacular, with thousands of songbirds moving through the riparian forest in April and early May, and fall migration is highlighted by the arrival of 30,000 southbound Turkey Vultures that roost in the riparian forest during September and October. Research at the Kern River Preserve has been ongoing since the 1980s when The Nature Conservancy was the owner, and has resulted in numerous publications on the Yellow-billed Cuckoo, the Southwestern Willow Flycatcher and Turkey Vulture migration. Most of the current work is coordinated by the Southern Sierra Research Station at the Kern River Preserve.

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Conservation Issues

Parasitism by Brown-headed Cowbirds was a major threat to the riparian songbird community, which rebounded dramatically following the initiation of cowbird trapping in the 1990s (with the notable exception of Southwestern Willow Flycatcher reproduction). Periodic inundation of the western three miles of riparian forest by Isabella Reservoir remains a threat (mainly in wet years), as does over-grazing by livestock on private lands adjacent to the reserves (mainly in dry years). A growing threat involves the lowering of the water table associated with a shifting away from grazing and toward irrigated row crops valley-wide, which could have a serious effect on the riparian habitat of the IBA (fide B. Barnes).

Ownership

Though much of the 10,000-acre valley floor is privately-held by large cattle ranches, several thousand acres are protected as conservation lands from the eastern edge of Isabella Reservoir east past the community of Onyx, including South Fork Wildlife Area (1200 acres, USFS) at the eastern edge of Lake Isabella; the Kern River Preserve (1100 acres, National Audubon Society) just to the east; and Canebrake Ecological Reserve (1300 acres, DFG) beyond Onyx.

Habitat

The South Fork Kern River Valley, about 15 miles in length, contains elements of all of these ecological zones, as well as one of the largest and best-preserved examples of lowland riparian woodland (Fremont Cottonwood-willow) in the state. Other major habitat communities include Joshua Tree woodland, wet meadow, freshwater marsh, Mojave Desert scrub, desert chaparral, and annual grassland.