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Monitoring Least Bell's Vireo (Vireo Bellii Pusillus) and Comparing

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Monitoring Least Bell’s Vireo (Vireo bellii pusillus) and comparing breeding landbird populations at the Dos Rios Ranch restoration site and San Joaquin River National Wildlife Refuge 2015-2016 Report prepared for River Partners, in support of habitat restoration work funded by the Central Valley Project Conservation Program. Final Report in fulfillment of grant agreement R12AP20047. November 2016 Kristen E. Dybala, Robert G. Walsh, and Nathaniel E. Seavy Monitoring Least Bell’s Vireo (Vireo bellii pusillus) and comparing breeding landbird populations at the Dos Rios Ranch restoration site and San Joaquin River National Wildlife Refuge 2015-2016 November 2016 Point Blue Conservation Science Kristen E. Dybala, Robert G. Walsh, and Nathaniel E. Seavy Acknowledgements This project was made possible by funding from the Central Valley Project Conservation Program. Suggested Citation Dybala KE, Walsh RG, Seavy NE. 2016. Monitoring Least Bell’s Vireo (Vireo bellii pusillus) and comparing breeding landbird populations at the Dos Rios Ranch restoration site and San Joaquin River National Wildlife Refuge 2015-2016. Point Blue Conservation Science, Petaluma, CA. This is Point Blue Contribution No. 2101. Point Blue Conservation Science – Point Blue’s 140 staff and seasonal scientists conserve birds, other wildlife and their ecosystems through scientific research and outreach. At the core of our work is ecosystem science, studying birds and other indicators of nature’s health. Visit Point Blue on the web www.pointblue.org. Cover photo credit: Robert G. Walsh Table of Contents EXECUTIVE SUMMARY .................................................................................................................... 1 INTRODUCTION ............................................................................................................................... 2 MONITORING OBJECTIVES .............................................................................................................. 3 METHODS AND RESULTS ................................................................................................................. 4 Study areas and sampling design ................................................................................................ 5 Bird surveys and vegetation monitoring at point count stations ............................................... 5 Riparian landbird response to restoration ................................................................................. 6 Least Bell’s Vireo monitoring .................................................................................................... 11 RECOMMENDATIONS ................................................................................................................... 15 PROJECT DATA .............................................................................................................................. 17 Data management .................................................................................................................... 17 Other products .......................................................................................................................... 17 REFERENCES .................................................................................................................................. 18 APPENDICES .................................................................................................................................. 22 Appendix A. Point count coordinates ....................................................................................... 22 Appendix B. Vegetation data and metadata ............................................................................ 22 Appendix C. Least Bell’s Vireo media and coordinates ............................................................ 22 Appendix D. PowerPoint slides of key results .......................................................................... 22 Appendix E. Annotated bird list, 2016 ...................................................................................... 23 P a g e | 1 EXECUTIVE SUMMARY This report describes the results of our 2015-2016 efforts to (1) document any evidence of colonization by Least Bell’s Vireo on Phase 1 of the Dos Rios Ranch restoration or recolonization on the adjacent San Joaquin River National Wildlife Refuge (SJRNWR), Stanislaus County, California, and (2) assess the response of the overall breeding landbird community to the Dos Rios Ranch restoration phase 1 and how it compares to the breeding bird community using the older restored riparian habitat in the SJRNWR. During spring of 2015 and 2016, we surveyed birds at 39 point count stations distributed across restoration sites at Dos Rios Ranch and SJRNWR, including 2 visits per year to each station for a total of 156 point count surveys. Vegetation was surveyed at each point count station in each year, and an additional 107 hours were spent on targeted searches for, and monitoring of, Least Bell’s Vireos. A single territorial male Least Bell’s Vireo was observed at the SJRNWR for two weeks in June of 2016. To our knowledge, this was the only record of the species from the entire Central Valley for the 2016 breeding season. A search for all Bell’s Vireo records from all of Central and Northern California produced an estimated average of just 1.3 individual vireos detected per year over the past several decades. Thus, in addition to requiring suitable riparian habitat, successful colonization of Dos Rios Ranch and SJRNWR by Least Bell’s Vireo will depend on whether more birds disperse out of southern California. We compared the species richness and breeding densities of landbirds at the Dos Rios Ranch restoration Phase 1 (1-2 years old) to restoration sites at SJRNWR, grouped into sites that were 3-6 years old and 9-12 years old. Species richness and the breeding densities of six Central Valley Join Venture (CVJV) riparian focal species at Dos Rios Ranch were already similar to that of the 3-6 year old SJRNWR sites. Further, Dos Rios Ranch has already achieved the CVJV breeding density objective for Song Sparrows. Using the data from SJRNWR and Dos Rios together, we found that bird species richness was correlated with tree, shrub, and ground cover, as well as canopy height, suggesting that species richness will continue to increase as restoration plantings mature. Finally, we make several recommendations: (1) Continue to evaluate restoration success using a broad suite of riparian species, which can provide broad insights into changes in the overall condition of riparian habitat. (2) Maximize the chances of vireo colonization by ensuring that restored sites are maturing into the vegetation structure most suitable for Least Bell’s Vireo. (3) Develop a plan for monitoring any future vireo nest attempts, including obtaining and maintaining permits. (4) Consider providing habitat for Tricolored Blackbirds at Dos Rios Ranch. (5) Consider monitoring black rats, Brown-headed cowbirds, or other species that could have negative impacts on riparian restoration success. (6) Evaluate whether current restoration designs are climate-smart and capable of adapting to projected climate conditions. (7) Continued to restore riparian vegetation and improve floodplain connectivity to provide important habitat for riparian-dependent species, as well as to provide the many other benefits of improved riparian ecosystem function, such as reducing flood risk, sequestering carbon, and contributing to local economies. P a g e | 2 INTRODUCTION Riparian ecosystems are hotspots of avian diversity in the western United States (Knopf et al. 1988; Knopf and Samson 1994). The Central Valley of California in particular provides essential breeding and wintering habitat, migration stopover areas, and corridors of dispersal for riparian- associated birds (Humple and Geupel 2002; Latta et al. 2012; Cormier et al. 2013; Dybala et al. 2015). Sixty-two species of landbirds have regularly bred in Central Valley riparian forests over the last two decades (Point Blue Conservation Science unpublished data) and many others use riparian areas during portions of their life cycle. Loss of riparian habitat may be the most important cause of population declines among landbird species in western North America (DeSante and George 1994), including the decline and extirpation of many riparian species once common in the Central Valley (RHJV 2004; CVJV 2006; Dybala et al. In press). While the populations of many riparian songbirds have declined in the Central Valley, the extirpation of the Least Bell’s Vireo (Vireo bellii pusillus) is one of the most dramatic examples (Howell et al. 2010). The Least Bell’s Vireo was once common in riparian habitat of coastal Southern California and the Central Valley (Goldman 1908; Grinnell and Miller 1944). However, by the late 1970s, the species had declined dramatically; extensive surveys in the Central Valley failed to detect a single individual (Goldwasser et al. 1980). As a result, the Least Bell’s Vireo was listed as state endangered in 1980 and federally endangered in 1986. In 1986, only 300 pairs remained with the majority in San Diego, Riverside, and Santa Barbara counties and eight or fewer pairs or territorial males within each of San Bernardino, Riverside, Orange, Los Angeles, Ventura, Inyo, and Monterey counties (USFWS 1998). More recently, the outlook for the Least Bell’s Vireo has been improved by extensive efforts to restore habitat and boost reproductive success. As a result of these
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