Important Bird Areas

Lower Colorado River Valley

California

This IBA contains some of the most important habitats for California's most imperiled avifauna of the wetlands and riparian thickets of the lower Colorado River Valley. The original riparian and wetland habitat of the Colorado River has been extensively altered. The best habitat is found at the Nevada/Arizona/California border at Fort Mohave and then to the south between Blythe, CA and Yuma, AZ. However, scattered patches of habitat elsewhere are probably critically important for birds in this arid landscape. The riparian thicket and woodland is generally dominated by tamarisk, but some areas have developed native willows and Arrowweed.

Starting in the north, the Fort Mohave area is just south of the Nevada border to the town of Needles. It includes the Fort Mohave Indian Reservation and Havasu NWR (part of the lower Colorado River refuge complex). Although most of the habitat here, mainly Honey Mesquite bosque, was cleared during the 1990s, it still supports populations of sensitive species on the eastern side of the "Needles Hwy" and along the main stem of the Colorado River.

To the south, some of the finest remnants include Hall Island on the southern end of Arizona's Parker Valley and Big Hole Slough, which protects about 100 acres of riparian and freshwater marsh habitat along an old oxbow of the River. Agricultural fields in the area are considered part of this IBA due to their current importance for birds. On the southern end of Blythe's Palo Verde Valley, Cibola NWR straddles the Arizona/California border. Key habitats include Three Finger Slough (40 acres) and Walker Lake (c. 350 acres of riparian), both impoundments with freshwater marsh and willow-tamarisk thickets. Moving south, nearby Draper Lake within Imperial NWR (c. 200 acres of riparian) is another former oxbow with similar habitat but lacks levees which permits the river to form natural meanders and oxbows. To the South, Picacho State Recreation Area supports another 350 acres of habitat.

Ornithological Summary

The birds of this IBA have been monitored by Robert L. McKernan and Gerald Braden of the San Bernardino Co. Museum since 1996. The species along the river seem to vary somewhat predictably in their occurrence, with some (e.g. Lucy's Warbler, Yellow-breasted Chat) being fairly widespread and others (Elf Owl, Yellow-billed Cuckoo) being very rare, and often occurring only with large aggregations of the commoner taxa. Big Hole Slough supports one of the few California occurrences of Northern Cardinal (RM), and the Fort Mohave area is known as one of the last holdouts for Elf Owl in California (Rosenberg 1991), which, unfortunately, may already be extirpated (5 pr. in 1987, 1 bird as recently as 1997, fide R. McKernan). Because the river is emerging as one of the most important corridors in the state for northbound migrants in spring, the agricultural fields to the west of Blythe (esp. along Lovekin Blvd.) support exceptionally high numbers of migrant shorebirds when flooded (e.g. up to 10,000 Whimbrel in spring, RM). Long-billed Curlew is also found in migration and winter in large flocks. The fields in this area host one of just two large aggregations of Mountain Plover left in southern California (several hundred birds), the other being in similar habitat in the Imperial Valley. Finally, Mayflower County Park, just north of Big Hole, as emerged as one of the premiere spots along the river to observe songbird migration, with large numbers moving through both spring and fall (R. Higson, pers. comm.). The Cibola area seems to support the most complete suite of riparian breeders, as well as impressive numbers of migrant and wintering waterfowl (including occasional flocks of wintering Sandhill Crane). Most of the other riparian habitat areas mentioned above have supported all of the sensitive riparian species within the last 10 years, though not all occur at every site each year. One other species of note, Harris' Hawk, maintains a small breeding population just across the river from Ferguson Lake, and could be expected to eventually colonize the habitat toward the southern end of this IBA (RM).

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Conservation Issues

To say that this area is critically threatened almost misrepresents the conservation urgency here. In general, the populations of sensitive species seems to fluctuate from year to year, with taxa like Yellow-billed Cuckoo and Willow Flycatchers breeding for a few years and then vanishing. This underscores the need to thoroughly survey all patches of riparian habitat in the region and speaks to the importance of even small patches for birds that may be displaced by such events as water level manipulation, fires and local habitat clearing. Current threats include ongoing direct habitat destruction (e.g. Senator Wash just north of Imperial Dam is regularly scraped clean of vegetation) and the general problems associated with fragmented riparian habitats (exotic plant invasions, cowbird parasitism). Encouragingly, some of the most sensitive species (incl. Yellow-billed Cuckoo) have partially adapted to nesting in tamarisk, given appropriate old-growth structure (RM). The habitat in this portion of the Lower Colorado River Valley, though productive, is very limited compared to historic extent, and could be greatly enhanced with additional restoration of public lands along the main stem of the river (e.g. Goose Flats Wildlife Area, just south of I-10). As with other agricultural areas of the state attractive to birds, changing cultivation/irrigation practices (e.g. conversion of alfalfa fields to row crops) could have dramatic effects on the birds using the habitat (e.g. Mountain Plover).

Habitat

This IBA draws attention to what is arguably the most important area for California?s most imperiled avifauna ? that of the wetlands and riparian thickets of the lower Colorado River Valley. The original riparian and wetland habitat of the Colorado River has been extensively altered, but particularly the downstream portions, including all of that within California. Most of the best habitat is (fortunately) located off the beaten path, even for this remote corner of California, with ?nodes? of habitat at the Nevada/Arizona/California border at Fort Mohave and then to the south between Blythe, CA and Yuma, AZ. However, scattered patches of habitat elsewhere (e.g. near Headgate Dam just north of Parker, AZ) are probably critically important for birds in this arid landscape. The riparian thicket and woodland here is now generally dominated by tamarisk, but some areas have developed a very significant component of native willows and Arrowweed.