California has a rich and diverse avifauna, with over 500 species residing or using parts of the state during the course of a year, some as residents and others as migratory species. California is second only to Texas in North American diversity. The topographic and habitat complexity of the state has fostered geographic and ecological subdivisions, leading to high species diversity, rich avifaunal communities, and endemism at the species and subspecies levels. A now-classic treatment of California’s avifauna is Grinnell and Miller’s The Distribution of the Birds of California. Miller updated this in 1951, and in this update he divided the state’s avifauna into four faunal groups: these were a Boreal avifauna and three Austral faunas, the Great Basin, Sonoran, and Californian. While Miller himself considered these to be “admittedly somewhat arbitrary”, they have served ornithologists fairly well. Quail Ridge lies within the Californian avifauna, and specifically within the Clear Lake Geographical Area on a peninsula jutting into the Berryessa Reservoir. The Californian avifauna is the richest of the four (at least within the state’s boundaries) and includes the majority of endemic forms found in the state. Most of the species in the Californian avifauna are year-round residents, although many exhibit seasonal elevational shifts, and the region supports a rich transient diversity.
The Quail Ridge Reserve lies in the heart of the Northern California coastal oak and chaparral zone, and its steep topography and damp canyons afford diverse habitats supporting an equally diverse avifauna. Since surveys began in 2000, 132 species of birds have been recorded from the Reserve. Of that total, 49 species have either been confirmed, or are suspected, to breed on the Reserve. The species diversity is among the highest recorded in the UC Natural Reserve System in similar habitats. Two other nearby Reserves, Stebbins Cold Canyon and McLaughlin, have similar habitats to Quail Ridge but have a more limited avian diversity of 110 and 120 species respectively. Why does Quail Ridge’s diversity appear to be higher? The answer lies with the “lake effect” and dense black oak canyon bottoms that are moist and support numerous species in an otherwise dry setting.
Seasonal Nature of Birds of Quail Ridge
California has a Mediterranean climate characterized by mild winters and hot summers. As a result the region sees several temporal patterns in avifaunal diversity that affect species composition. These patterns and species of birds can be separated into four groups:
Residents – those species that are found on the Reserve year-round;
Winter visitors – those species that migrate to the Reserve in fall to spend their non-breeding (winter) months and then depart to their breeding grounds either in northern latitudes or higher elevations;
Summer visitors (neotropical migrants) – those species that migrate to the Reserve in spring to breed and then depart to southern latitudes;
Transients – those species that pass through the Reserve in spring and fall en route to and from their breeding grounds and wintering grounds.
Many of the resident species are among the most characteristic and conspicuous species at Quail Ridge. These include species such as Western Scrub-Jay, Oak Titmouse, Anna’s Hummingbird, California Towhee, Wrentit, and California Thrasher. The winter visitors generally arrive from mid-September through October and remain on the Reserve through April. Winter visitors are dominated by flocks of sparrows (White-crowned, Golden-crowned, Fox) and insectivores such as kinglets (Ruby-crowned and Golden-crowned), Chestnut-backed Chickadee, and frugivores that concentrate their foraging on toyon berries (Hermit and Varied Thrushes, American Robins, Cedar Waxwings). Summer visitors start their arrival in early March with the first returning birds comprised of swallows (Cliff, Violet-green, and Tree). These are followed in April and May by several neotropical migrants including Black-headed Grosbeak, Cassin’s Vireo, Orange-crowned Warbler, and Pacific-slope Flycatcher.
From mid-April through early June these gifted songsters enliven the dawn chorus of the chaparral and oak woodlands. Summer visitors arrive to breed and depart quickly as the canyon water supplies dwindle by mid-summer. Most depart by mid- to late July each year.
From late March through early June and again from early September through mid-October, the transient species move through Quail Ridge. During these spring months the combination of residents, returning breeders, and transient species make exciting bird watching as the Reserve’s avian diversity reaches its peak. The combination of flycatchers, warblers, vireos, thrushes, and sparrows can be overwhelming to observers. This migration is timed well to take advantage of the numerous insect blooms, particularly geometrid moths associated with blue and live oaks.
Characteristic of insectivorous birds in winter is the formation of mixed-species foraging flocks. These flocks center on core species that include bushtits and kinglets. Wrens, chickadees, creepers, small woodpeckers, warblers, and vireos key on these aggregations, as they move together in tight groups through the forest canopy. Finding one of these flocks can be an exciting and rewarding activity for bird watchers visiting the Reserve because one never knows what unusual species might be found.
Part of California’s inner coastal region, on the edge of the Central Valley, Quail Ridge offers a unique temperate mix of habitats with a nearby freshwater source in Berryessa Reservoir. The gradations of chaparral, live oak, riparian woodland, and lakeshore constitute a host of microhabitats and ecotones that support Quail Ridge’s diverse avifauna. Birds use these habitats for nesting, foraging, and territorial displays. Congeners partition resources based on tree species and/or spatially within the canopy or undergrowth. For example Hutton’s and Cassin’s Vireos partition resources in the oak woodlands with Cassin’s Vireo more abundant in black oak woodlands than in live/blue oak woodlands (4.67 birds/ha vs. less than .5 birds/ha respectively), and Hutton’s Vireo more abundant in the live/blue oak habitat but also found in black oak habitats (2.65 vs. 2.02 birds/ha respectively). How the two species partition the black oak woodland is still unknown (Engilis, pers. comm.).
Each major plant community supports a characteristic assemblage of bird species. Chaparral, which dominates dry, western- and southern-facing slopes of Quail Ridge, often hosts localized, intermittent brush fires and landslides that maintain a mosaic of young and old successional stages dominated by brushy vegetation. A wide variety of small and ground-dwelling birds thrive here, including many California endemics – California Thrasher, California Towhee, and Wrentit. Blue-gray Gnatcatcher, Anna’s Hummingbird, Sage Sparrow, Rufous-crowned Sparrow, Common Poorwill, and California and Mountain Quail also use chaparral’s dense groundcover.
Black oak woodlands dominate north-facing canyons at Quail Ridge and host birds that favor a denser canopy and relatively moist surroundings. Species such as the Pacific-slope Flycatcher, Red-breasted Sapsucker, Cassin’s Vireo, Warbling Vireo, and Pileated Woodpecker reside almost exclusively in these areas of the Reserve. Other inhabitants include the Great-horned Owl, Dark-eyed Junco, Lawrence’s Goldfinch, and Winter Wren.
Live and blue oaks are interspersed with black and valley oaks in Decker Canyon, but they also extend further up in elevation to meet the ridges. This transition from covered to more open woodland facilitates use by species that prefer either setting or those that require a variety of habitats. Oak Titmouse, Hutton’s Vireo, Western Screech Owl, Lesser Goldfinch, Ash-throated Flycatcher, Blue-gray Gnatcatcher, and California Towhee are some that occur in this more open forest. Open woodland is characterized by larger forest clearings, which are the favored by Western Kingbirds, Wild Turkeys, and Western Bluebirds. Habitat generalists like the Spotted Towhee are widespread from the ridge to Decker Canyon.
Distinct from these upland communities, the waters and shoreline of Berryessa Reservoir attract a number of aquatic birds. Ospreys, Bald Eagles, Belted Kingfishers, grebes, cormorants, mergansers, and herons rely on the reservoir’s supply of fish. Gulls and Spotted Sandpipers occupy the shores, and on the open waters float ducks, geese, and coots. Western and Clark’s Grebe, two closely related congeners, appear to partition the lake arms – Clark’s Grebes restricted predominately to Wragg Canyon and Western Grebes to other arms and open water around the Quail Ridge peninsula.
Birds from many families – including woodpeckers (Picidae), wrens (Troglodytidae), Ash-throated Flycatchers (Tyrannidae), Oak Titmice (Paridae), Western Bluebirds (Turdidae), Violet-green Swallows (Hirundinidae), and White-breasted Nuthatches (Sittidae) at Quail Ridge – nest in cavities that they find, usurp, or excavate themselves. Cavities afford more shelter from environmental conditions and predators than most open nests, which may explain why cavity-nesters often lay white, unmarked eggs. However, successful breeding is strongly limited by the number of appropriate and available cavities, resulting in high levels of competition for nest sites. Some species favor snags, and others can only excavate old, rotting trees with softened interiors. With such high resource dependence, native cavity-nesting populations are often threatened by more aggressive introduced species including the European Starling and the Brown-headed Cowbird. Starlings steal holes particularly from Western Bluebirds, while cowbirds parasitize a wide variety of species, often ejecting or eating one of the host’s eggs.
California’s Endemic Birds
Richly unique in vegetation, the California floristic province extends from the southwestern edge of Oregon to northwestern Baja California and includes more than 70% of California. A Mediterranean climate of hot, dry summers and cool, wet winters facilitates the persistence of a large diversity of ecosystems: coastal sage scrub, prickly pear shrubland, sagebrush steppe, coastal dunes, salt marshes, chaparral, and multiple types of oak woodland and coniferous forest. The mild climate, variety of habitats and landforms, and sheer size of the region make this province host to one of the richest arrays of endemic species in North America.
With oak and chaparral habitats characteristic of the floristic province (Habitat Use section), Quail Ridge provides both breeding and foraging grounds for four of the eleven bird species endemic to the floristic province: California Thrasher, California Towhee, Nuttall’s Woodpecker, and Oak Titmouse. In addition, Lawrence’s Goldfinches breed only in the oak woodlands of California. Although this species is also found in Arizona and Mexico outside of Baja – and therefore is not a California province endemic in the strictest sense – it may be termed a “breeding endemic.”
Incipient speciation events have probably increased the number of endemic subspecies in the California floristic province. Geographical barriers, most notably the Sierra Nevada, have facilitated separation of populations within the same species. If the isolation persists for an evolutionarily significant period of time, two distinct species may result. Populations diverge genetically preventing interbreeding were they rejoined. Newly developed pre-zygotic isolating mechanisms such as behavioral and morphological changes may further impede hybridization in the wild. Although difficult to confirm, such events seem to have occurred with multiple sister species, including the California and Canyon Towhees, Yellow-billed and Black-billed Magpies, and Nuttall’s and Ladder-backed Woodpeckers.
The case of the Oak Titmouse and Juniper Titmouse may be reviewed as an example of such speciation. The ranges of the two are clearly separated by the Sierra Nevada-Cascade ranges (left), a feature that discourages transit with its high altitudes and freezing temperatures. Formerly known together as the Plain Titmouse, these birds were only recently divided into separate species based on morphology, genetics, and vocal characters.
When geographical separation has taken place for a shorter period of time, the process of speciation may be ongoing and much more difficult to confirm. This phenomenon likely occurs quite frequently at local scales and can be disrupted when animals from either population hybridize and thus remix the gene pool. Researchers currently are working to identify species limits genetically, geographically, and morphologically for birds such as the Sage Sparrow, Rufous-crowned Sparrow, and Western Scrub Jay, all with disjunct populations. Such delineation has important implications for species conservation.