Audubon: Birds & Science

Glenn Olson, Executive Director of Audubon CA, volunteering by planting willows to create riparian habitat at Kern

Site Description:

The Kern River is one of the major rivers of the Sierra Nevada; its watershed extends from the highest point in the state south and west into the southern San Joaquin Valley. There are two main arms of the Kern, the North Fork and South Fork, which come together in Lake Isabella. Lake Isabella 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 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 by the U.S. Army Corps of Engineers, U.S. Forest Service, National Audubon Society, and the California Department of Fish and Game.

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

For IBA map, click here.

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. 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. 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 support large numbers of nesting Tricolored Blackbirds, 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 research 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).