PRINCIPAL INVESTIGATOR: Barbara Kus, San Diego Field Station

BACKGROUND AND JUSTIFICATION: Riparian habitat supports more species of birds, as well as other wildlife, than any other habitat type in the southwest. This is particularly true in southern California, where riparian woodlands provide a literal oasis in an otherwise arid landscape for some 140 species of birds, one third of which are riparian obligates. Riparian habitat is also one of the State’s most endangered habitats, with less than five percent of the woodlands present at the time of statehood remaining. It was inevitable that losses of this scope and magnitude lead to declines in many riparian species, bringing some, including the least Bell’s vireo (Vireo bellii pusillus) and the southwestern willow flycatcher (Empidonax traillii extimus), to the brink of extinction. These two endangered species are the focus of this investigation.

The least Bell's vireo is a migratory songbird dependent upon riparian habitat for nesting. Once widespread and abundant throughout riparian woodlands in California and northern Baja California, Mexico, the species has undergone a precipitous decline in numbers during the past 40 years, and by 1986 numbered just 300 pairs statewide. This decline has been attributed to the loss and degradation of riparian habitat throughout the vireo's range, as well as to the expansion in range of the brown-headed cowbird (Molothrus ater), a brood parasite, to include the Pacific coast of North America. In response to the dramatic reduction in numbers of least Bell's vireos in California, the California Fish and Game Commission listed the species as endangered on June 27, 1980, under the California Endangered Species Act of 1970. The species was subsequently listed as federally endangered by the U.S. Fish and Wildlife Service (USFWS) on May 3, 1986, under the Endangered Species Act of 1973. These designations afford procedural process for protection of the vireo under numerous state and federal laws.

During the decade since listing, the vireo population in southern California has rebounded, largely in response to cowbird control and nest monitoring programs which have effectively reduced the impact of parasitism on vireo productivity. As of 1998, the statewide vireo population was estimated to be approximately 2000 males. Roughly half of the vireos in California occur at Camp Pendleton, and the majority of the remainder occur along half a dozen major drainages, including the Tijuana, Sweetwater, San Diego, San Luis Rey, and Santa Ana.

The southwestern willow flycatcher is one of four subspecies of the willow flycatcher in the United States, with a breeding range including southern California, Arizona, New Mexico, extreme southern portions of Nevada and Utah, and western Texas. Like the least Bell’s vireo, the southwestern willow flycatcher is an obligate riparian breeder, and like the vireo has declined in response to widespread habitat loss and cowbird parasitism. By 1993, the species was believed to number approximately 70 pairs in California in small disjunct populations. The southwestern willow flycatcher was listed as endangered by the State of California in 1992 and by the U.S. Fish and Wildlife Service in 1995.

Unlike vireos, with which they share an extreme vulnerability to habitat loss and parasitism, willow flycatchers in California have failed to increase in the wake of intensive management efforts to reduce these threats. Outside of two populations numbering fewer than two dozen pairs each, most breeding willow flycatchers in San Diego County occur as single, isolated pairs.

The research described in this study plan extends research conducted by the P.I. since 1986 on the distribution, habitat requirements, nesting ecology, wintering distribution, foraging behavior, singing behavior, and demography of the least Bell’s vireo, and expands previous investigations of the distribution of willow flycatchers to include breeding ecology and dispersal. The focus of the current project is to further study the impacts of cowbird parasitism on vireos by analyzing nesting success in a population where cowbird control has been discontinued, and to add to our data base on demography by monitoring colorbanded individuals of known history. In addition, we will collect data on turnover rates of our breeding populations as preparation for a future study comparing turnover and recruitment of large and small vireo populations. Our goals with regard to willow flycatchers include collecting data on current distribution and nesting success, and facilitating long-term studies of dispersal and demography through colorbanding and monitoring individual birds. These data will provide information required by the U.S. Fish and Wildlife Service, California Department of Fish and Game, and other resource agencies to develop sound conservation and recovery plans for these species. In particular, they allow development of realistic population viability analyses for each species, which form the basis for assessing species persistence under a variety of management scenarios. They will also provide the first information comparing nesting success of vireos in areas managed and unmanaged for cowbirds, and shed light on the potential of large, recovering populations to withstand parasitism in the absence of annual cowbird control. These findings can in turn be used to modify or replace, as appropriate, current control measures with alternatives that entail less cost and human intervention while still maintaining stable or growing host populations.

OBJECTIVES:

Least Bell's Vireos

1. Document the current distribution and composition of breeding populations in two long-term study areas, the San Luis Rey River and Pilgrim Creek.

2. Document rates of parasitism and nest success in a region of the study area unmanaged for cowbirds ("east San Luis Rey"), and compare to (a) sites with active cowbird trapping and removal ("west San Luis Rey" and Pilgrim Creek), and (b) the same site in previous years with trapping.

3. Identify differences in the vegetation structure surrounding parasitized and nonparasitized nests.

4. Determine lengths of residency and turnover rates for breeding individuals through resighting of individually colorbanded birds.

5. Add to our long-term data base of age- and sex-specific survival rates, productivity, dispersal, and other demographic parameters through monitoring individually colorbanded birds.

Southwestern Willow Flycatchers

1. Document the current distribution and composition of breeding populations at the San Luis Rey River, Pilgrim Creek, and San Pasqual Valley.

2. Monitor nesting activity to document nesting success and rate of parasitism.

3. Colorband adults and nestlings to monitor site fidelity, return/survival rates of adults and young, within season movement of adults, and dispersal.

STUDY AREA: Surveys and monitoring are conducted at a number of drainages throughout San Diego County, California, with an emphasis on the San Luis Rey River and its tributaries (e.g. Pilgrim Creek), and the San Dieguito River and Santa Ysabel Creek in San Pasqual Valley. The San Luis Rey River study site supports approximately 150 least Bell’s vireo territories, and approximately two dozen willow flycatcher territories. Pilgrim Creek supports approximately 25 vireo and one flycatcher territories, while the San Pasqual Valley supports two to four flycatcher pairs and roughly 150 vireo territories. This study plan includes monitoring of only flycatchers at the latter site.