Utility of Riparian Corridors to Fragmented Songbird Communities

What species: Anna’s Hummingbird (Turdus migratorius), Bewick’s Wren (Thryomanes bewickii), Black Phoebe (Sayornis nigricans), Blue-headed Vireo (Vireo solitarius), Bullocks Oriole (Icterus bullockii), Bushtit (Psaltriparus minimus), California Quail (Callipepla californica), California Towhee (Pipilo crissalis), Chicadee (Poecile rufescens), Chipping Sparrow (Spizella passerina), Downy Woodpecker (Picoides pubescens), House Finch (Carpodacus mexicanus), House Wren (Troglodytes aedon), Hutton’s Vireo (Vireo huttoni), Lesser Goldfinch (Carduelis psaltria), Northern Flicker (Colaptes auratus), Nuttall’s Woodpecker (Picoides nuttallii), Oak Titmouse (Baeolophus inornatus), Orange-crowned Warbler (Vermivora celata), Oregon Junco (Junco hyernalis), Pacific-slope Flycatcher (Empidonax difficilis), Pileated Woodpecker (Dryocopus pileatus), Spotted Towhee (Pipilo maculates), Violet-green Swallow (Tachycineta thalassina), Warbling Vireo (Vireo qilvus), Western Bluebird (Sialia mexicana), Western Kingbird (Tyrannus verticalis), White-breasted Nuthatch (Sitta carolinensis).

Where: Alexander and Sonoma Valleys in Sonoma County, CA.

Who: Katie Meiklejohn in association with Columbia University and the Wildlife Conservation Society.

When: April, 2006 – present

Why (threats): Human encroachment in the form of vineyard expansion has led to widespread habitat loss and fragmentation in northern California. This expansion threatens biodiversity, particularly species such as many songbirds that rely on continuous habitat for survival.

Agricultural lands often function as barriers to dispersal, preventing migrations, cutting-off important food or habitat sources, and inhibiting gene flow between populations. Conservation biologists have proposed the use of habitat corridors as a means of improving connectivity in highly fragmented landscapes or as important areas of breeding habitat for other species, but more data are needed before conclusions can be drawn about the efficacy of corridors at improving population viability.

The highly fragmented oak woodland-vineyard landscape of northern California offers a unique opportunity to assess the utility of habitat corridors to a variety of passerine species.

Current Scientific or Conservation Problem this Project Addresses: The purpose of this study is to assess the value of riparian corridors to breeding songbird populations in order to add to existing scientific theory on the ability of habitat corridors to combat the negative effects of landscape fragmentation. These data will provide local and regional managers and farmers with information pertinent to the design of functional agro-ecosystems.

Current Goals: This project is currently in the data analysis stage. I am working in collaboration with statisticians from the Wildlife Conservation Society (WCS), and hope to publish my results next year.

Next Steps: Conservation corridors have great potential to improve landscape connectivity for a range of species, but different organisms will use corridors in different ways. Understanding these differences is critical to the design of effective corridors. My research is based on some existing work by Jodi Hilty of the WCS North America Program who collected presence-absence data for carnivores and songbirds in Sonoma County during her dissertation research. In collaboration with Jodi I hope to combine my results with this presence-absence data to assess the differences that might exist between carnivore and passerine communities within these corridors.

Project Description and Current Status: Grape production has been widely successful in northern CA due to the Mediterranean climate that characterizes this region of the country. Northern California is also known for its endemic oak forests and rich biodiversity but both are becoming scarce as thousands of acres continue to be converted to agriculture.

Riparian corridors in Sonoma County have the potential to counteract some of the negative ecological impacts that result from land conversion and habitat fragmentation by improving landscape connectivity. However, the factors that make corridors effective to individual species are not well understood, and many existing studies lack experimental components or take a single-species approach.

In contrast, this study takes advantage of natural conditions to compare the relative abundance and reproductive success of at least 30 native Californian songbirds across three experimental categories of corridor width. The results of this study will thus contribute a unique perspective to the scientific literature on the conservation value of habitat corridors to songbirds.

For birds, corridors have the potential to not only serve as conduits for movement, but also as breeding habitat. To date, studies examining the utility of corridors for breeding bird populations have produced inconclusive results. Some studies show elevated levels of nest predation and parasitism, and an absence of interior specialists, while other studies document greater species richness and abundance among groups such as Neotropical migrants. The value of corridors to breeding bird populations will be influenced by factors such as nesting strategy, rates of nest predation and parasitism, and habitat quality. Some corridors might function as habitat sinks in which birds breed, but unsuccessfully, while other corridors will be sources in which reproduction is successful, and from which individuals are recruited to the population.

Previous work in the focal corridors of this study documented an increase in nest predators, such as domestic cats, possums and raccoons, in narrower corridors. Furthermore, studies have shown that nest parasites, such as brown headed cowbirds, increase in abundance along habitat edges. Thus it is possible that in Sonoma County, narrow corridors function as habitat sinks and that there is a threshold width below which preserving habitat along creeks is not useful to the persistence of songbird populations.

To assess the utility of riparian habitat corridors to breeding populations of songbirds, I hypothesized that corridor width would influence reproductive success. Based on this hypothesis, I predicted that 1) birds breeding in denuded and narrow corridors would experience higher levels of predation compared to birds breeding in wide corridors and 2) birds breeding in denuded and narrow corridors would experience higher levels of nest parasitism by brown-headed cowbirds (Molothrus ater) compared to birds breeding in wide corridors.

Between April and July 2006 my field assistant and I conducted nest searches in six riparian corridors (two each of denuded, narrow and wide widths), standardized for length. Once a nest was located, we monitored its progress through to either fledging or failure, recorded its location relative to vineyard and creek, and classified the surrounding vegetation and the degree of cover it provided for each nest.

Funding: This project was supported by a grant from the National Aviary.

Acknowledgements: I would like to acknowledge my advisors Josh Ginsberg and Jodi Hilty, my field assistant Jennie Chaiet, and the generous vineyard landowners of Sonoma County who made available their property for the purposes of this project.