Vol. 18 No. 1 (1987)

When the Cooper Ornithological Society announced its annual meeting to be held in Davis, California, in the fall of 1986, the organizers requested topics for mini-symposia. Because one of the major management issues and conservation problems concerning birds in California and the arid West is the preservation and management of riparian ecosystems and their accompanying bird communities, I organized a steering committee to explore the possibility of conducting a symposium on this topic.

This steering committee consisted of Edward Beedy of Jones and Stokes Associates, Kathleen Franzreb of the U.S. Fish and Wildlife Service Endangered Species Office, and John Gustarson of the California Department of Fish and Game, Nongame Bird and Mammal Section.

The emphasis of this symposium is the management of endangered, threatened, and other sensitive bird species in western riparian habitats. Riparian areas are limited in their extent, yet are extremely productive and have well-documented wildlife values (Gaines 1977; Johnson and Jones 1977; Stevens et al. 1977; Warner and Hendrix 1984).

This paper examines the nature of riparian systems from historical and current perspectives, describes the importance of riparian habitats to sensitive bird species, and briefly explores the laws, policies, and regulations designed to protect these habitats.

Ecological values of riparian habitats have received much attention over the past decade. One issue confronting management agencies is the effect of intensive water management on riparian vegetation in the Southwest (e.g., the decline of native vegetation and the increase of exotic Salt Cedar (Tamarix chinensis); see Table 1).

Many bird species have declined in number or suffered extirpation along these riparian systems as habitat changes have occurred (Table 2). We examine breeding birds obligated to riparian habitats and draw qualitative conclusions on their status throughout their range below 1,524 meters elevation (Phillips et al. 1964; Ohmart 1982; Ohmart and Anderson 1982).

Historically, the California Yellow-billed Cuckoo (Coccyzus americanus occidentalis) was considered common in river bottoms throughout the western United States and southern British Columbia (Gaines and Laymon 1984). It began a drastic decline in numbers as the riparian forests on which it depended were removed for fuel and to make way for agriculture and grazing.

The Willow Flycatcher (Empidonax traillii) was formerly a common summer resident in California, breeding in riparian willow thickets. It has been extirpated from most of its California range and is currently under consideration for state Threatened or Endangered status (R. Schlorff, pers. comm.).

Most of the remaining populations occur in isolated mountain meadows of the Sierra Nevada and along the Kern, Santa Margarita, and San Luis Rey rivers (Remsen 1978; Serena 1982; Unitt 1987). The California Department of Fish and Game conducted a survey for Willow Flycatchers in six Sierra Nevada national forests and Yosemite National Park in 1982 (Serena 1982).

Willow Flycatchers (Empidonax traillii) have declined in California, and much of the state's population is now restricted to meadows in the Sierra Nevada (Harris et al. 1987; Serena 1982).

To understand the factors contributing to their decline and to develop recommendations to protect and enhance the flycatchers’ habitat, we studied a Sierra Nevada population of Willow Flycatchers in the late spring and summer of 1986.

In this paper, we present preliminary information on Willow Flycatcher nesting success, territory and habitat characteristics, and effects of livestock grazing. We conclude with some management recommendations based on this study and the results of a distributional survey also conducted in the summer of 1986 (see Harris et al. 1987).

The Least Bell's Vireo (Vireo bellii pusillus) is a small, gray, migratory passerine that feeds mainly on insects. The normal clutch of four eggs is incubated for about 14 days. The young remain in the nest approximately 10–12 days. The Least Bell's Vireo arrives in its breeding habitat from mid-March to early April and departs in late August or September for its wintering range in Baja California, Mexico.

The Least Bell's Vireo usually constructs its nest low to the ground, primarily in willow-dominated riparian habitats, but also uses a variety of shrubs, trees, and vines. Nesting is now largely restricted to small, remnant segments of willow-dominated habitats. Its precarious status prompted the U.S. Fish and Wildlife Service (FWS) (1986a) to officially designate it as an endangered species on May 2, 1986. The state of California classified the vireo as an endangered species in 1980.

In 1978, the total population of the Least Bell's Vireo (Vireo bellii pusillus) was estimated at 90 pairs (Goldwasser et al. 1980). With the expected listing of the subspecies as endangered by the California Fish and Game Commission, and interest in listing it at the federal level, information on the vireo’s breeding biology and population dynamics was needed.

No such data were available, and studies of the nominate subspecies (V. b. bellii) addressed only reproductive rates and nesting substrate, with data gathered from small samples over many years (Overmire 1962), or reported sample sizes too small (Mumford 1952; Nolan 1960; Barlow 1962) to be useful for analysis of population dynamics.

For five breeding seasons (1979–1983), I conducted a banding study of a Least Bell’s Vireo population at Gibraltar Reservoir, Santa Barbara County, California, in order to assess the population’s dynamics.

To promote the survival of the Least Bell's Vireo (Vireo bellii pusillus), the California Department of Transportation (Caltrans) began a program of trapping Brown-headed Cowbirds (Molothrus ater) on the Sweetwater River of San Diego County in 1986. The trapping was part of the mitigation required for a U.S. Army Corps of Engineers 404 permit to compensate for the removal of 1 acre of vireo habitat for construction of a bridge. The U.S. Fish and Wildlife Service (USFWS) provided details of the trapping scheme through an interagency cooperation agreement.

This paper addresses the present USFWS management method for reducing brood parasitism of vireos through trapping of cowbirds. Such trapping should manage both the Least Bell's Vireo and Brown-headed Cowbird effectively, but we contend that the present program does not fully address the behavior or ecology of the latter species.

Two subspecies of Brown-headed Cowbird (Molothrus ater) are now known to breed in California. The Sagebrush Cowbird (M. a. artemisiae) has probably always been a rare summer resident in the Great Basin portions of California and a rare winter visitor in the remainder of the state (Grinnell 1915; Mailliard 1927).

Although the abundance of this subspecies has increased, its distribution has remained constant.The status of the Dwarf Cowbird (M. a. obscurus), on the other hand, has changed remarkably during the past 120 years, and it is the latter subspecies that is apparently responsible for the substantial reduction in reproductive success of a number of Central Valley and Southern California passerines.

Throughout its Holarctic breeding range, the Bank Swallow (Riparia riparia) is generally regarded as a riparian species, although it has not been shown to be dependent on riparian vegetation. It is a colonial bird that nests in earthen banks and bluffs, as well as in sand and gravel pits.

Once locally abundant in lowland California (Grinnell and Miller 1944), the Bank Swallow has declined in numbers in recent years and no longer breeds in much of its former range (Remsen 1978). The Bank Swallow has a rather localized distribution in California along rivers, lakes, and ocean coasts (Grinnell and Miller 1944). We estimate that approximately 70–80% of California's remaining Bank Swallows nest along the Sacramento River.

Recently, much research has focused on the effect of forest fragmentation on so-called "forest interior" bird species. Forest interior birds are dependent for breeding habitat on the central portions of large forest tracts; a decline in these species has been associated with a reduction in the size and quality of remnant forests (Robbins 1979; Whitcomb et al. 1981; and others). Fragmentation creates a greater proportion of edge habitat, resulting in increased nest predation (Wilcove 1985) and brood parasitism (Brittingham and Temple 1983). Ranney et al. (1981) have described edge-related changes in forest vegetation.

The California Natural Diversity Data Base (Data Base) is an ongoing project of the Department of Fish and Game's Nongame-Heritage Program.

This paper presents background information on the Data Base, briefly describes the types of riparian-associated data collected and computerized within it, and explains the importance and usefulness of the Data Base for riparian ecosystem conservation.

Riparian ecosystems are the most productive—and possibly the most sensitive—of the various bird habitats in the arid and semiarid North American Southwest. The highest population densities of non-colonial nesting birds in North America were reported from riparian cottonwood (Populus fremontii) forests in central Arizona by Johnson (1971) and Carothers et al. (1974).