This mosaic of wet pastures, tidal wetlands, and two major estuaries (Mad River, Eel River) extends from McKinleyville north of Arcata south to the confluence of the Eel and Van Duzen rivers along Hwy. 101. One of the premiere natural estuaries in the state, Humboldt Bay transforms from a placid lagoon to an extensive mudflat rimmed with saltmarsh during low tide, with much of its habitat managed as Humboldt Bay National Wildlife Refuge. Just south of the bay, a vast system of tidal channels cuts through wet pastures and dairy farms along the Eel River. Historically prone to major flooding events in early spring, it has remained undeveloped, and is partially protected as the Eel River Wildlife Area (DFG). The Nature Conservancy's Lanphere-Christensen Dune Preserve, one of the finest examples of coastal dunes in the state, lies within this IBA, separating the bottomlands from the ocean. A significant amount of freshwater marsh habitat has been restored/recreated at Arcata Marsh, a progressive, city-owned wetland/water treatment facility on the northeast side of Humboldt Bay that has become a model for such efforts worldwide. Other key habitats in this area include the boggy willow thickets near the town of Fairhaven on the Samoa Peninsula, which are within BLM's Samoa Dunes Recreation Area, a popular off-road vehicle playground, and Indian Island off Eureka, which supports a large heron rookery. Long-term bird banding stations were established two decades ago adjacent to the Mad River Slough and the dunes preserve (Humboldt Bay Banding Lab) and, more recently, at the confluence of the Eel and the Van Duzen rivers.

Updated by Redwood Region Audubon, May 2008

Up to 80,000 shorebirds have been estimated at Humboldt Bay during peak days in spring and fall (Colwell 1994). The state's largest concentration of Brant (c. 25% of Pacific Flyway population, fide Sedinger et al. 1994) occurs here in spring, when 30,000 may feed on eelgrass in the bay. Both the tidal wetlands and agricultural fields provide roosting habitat for wintering shorebirds. Breeding Snowy Plovers occur here, nesting on gravel bars along the Eel River, adjacent beach, and along the south spit of Humboldt Bay. The beach at the mouth of the Mad River in the north also supports a small nesting population of plovers (R. Hewitt, pers.comm.). A large heron rookery on Indian Island is among the largest in the state, with another located at Teal Island on the southeast corner of the bay. Near-shore waters appear to be critical for non-breeding concentrations of Marbled Murrelet, which maintain one of the largest populations in the state within the Eel River drainage southeast of the bay (JH). Riparian habitat, somewhat limited on the immediate coast in this bioregion, is well developed along the Mad River in the Blue Lake area and along the Eel River. These rivers are also important foraging areas for Bald Eagle, which breeds just inland of the bay (Harris 1996). Several species are highly localized, particularly open-country species such as Ferruginous Hawk, and breeding Northern Harrier, Short-eared Owl and Grasshopper Sparrow. A breeding outpost of Tricolored Blackbirds forage on nearby dairy farms in the Fortuna area. For migrant passerines coming in off the Pacific, the willows near Fairhaven offer the first suitable foraging habitat in the area, and are probably the best place to observe fall songbird migration on the northwest coast. Up to 130,000 Aleutian Cackling Geese, a threatened species reduced to about 700 individuals in 1973, delisted in 2001, are wintering on pasture lands around Humboldt Bay.

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This IBA is a patchwork of public and private lands, with much of the latter in agricultural production (with clear benefits to certain bird species, such as wintering raptors and roosting shorebirds). Much of the Eel River Bottoms are still in private ownership and are therefore vulnerable to development and/or alteration. However, since this area is so flood-prone and is under active agricultural production, development seems unlikely at this point. Though the edges of the bay are reasonably well protected as a national wildlife refuge, the bay itself must contend with the effects of a major oyster-culture operation, which periodically dredges the eelgrass beds, and covers them with used shells (Harris 1996). Dune stabilization with non-native beach grasses and lupines has eliminated Snowy Plover nesting habitat. Many sensitive species depend on the non-wetland vegetation (grassland, riparian stringers), and maintenance of these habitats even on public lands has been a challenge to local conservationists. One recent decision to convert a large area of the Mad River Wildlife Area to cattle pasture (at the urging of local dairymen) led to the elimination of a major communal raptor roost (47 White-tailed Kites, 12 Short-eared Owls, fide D. Fix). Another grassland site conversion to wetland (Fay Slough Wildlife Area) has reduced Short-eared Owl wintering habitat (15 Short-eared Owls, fide C. Ogan) Similar management conflicts have occurred at the "Ocean Ranch" section of the wildlife area, one of the premiere shorebird sites (extensive mudflats) that DFG has proposed converting to diked impoundments (D. Fix, pers. comm.). The fact that the "Fairhaven Willows" are located within a major off-road vehicle area, Samoa Dunes Recreation Area, leaves them open to degradation, and they should be considered vulnerable to alteration unless given formal recognition for their habitat value. Further conversion from grazing to bulb farms has made land less available for shorebirds