Songbird Habitat Associations and Response to Disturbances in the Point Reyes National Seashore and Golden Gate Recreation Area

Holmes, Aaron; Humple, Diana; Geupel, Geoffrey; Gardali, Thomas

Brief Description: This report presents the findings of the data collected in the 1997 and 1998 field seasons and uses that data evaluate the effects of the Mt. Vision fire in songbirds in riparian forests. Full Description: In the spring of 1997, the Point Reyes Bird Observatory (PROB) began a cooperative assessment of songbirds with the Point Reyes National Seashore (PORE) and the Golden Gate National Recreation Area (GOGA), in riparian, coastal scrub and mixed hardwood forest habitats. Riparian monitoring was a continuation of, and elaboration upon, a Brown-headed Cowbird study initiated in 1995 by the Kern River Research Center (KRRC) with PORE and GOGA. Study sites within these major habitat types include those that were actively grazed and burned in the Vision fire of 1995. Our primary objective was to determine the distribution, relative abundance, and species diversity of songbirds using coastal scrub and riparian habitats of PORE and GOGA. A secondary objective of this project was to investigate habitat affinities and response if these species to fire and ongoing livestock grazing. In this report we present findings from census data gathered in the 1997 and 1998 breeding seasons. Data gathered in 1998 was used to model habitat associations and evaluate the effects of the Mt. Vision fire on breeding songbirds in riparian forests three years post-burn. This project has documented the importance of riparian areas on the lands managed by the National Park Service in coastal Marin County. Our results have linked habitat characteristics and specific vegetation structure to bird abundance and diversity. Neotropical migrant species such as Pacific-slope Flycatcher, Warbling Vireo, Wilson’s Warbler, and Black-headed Grosbeak, were all more abundant in areas where a greater number of tree species are present. Wider riparian forests with high numbers of tree species, and a variable shrub structure support the most diverse community of breeding birds. Other species such as Song Sparrow and Common Yellowthroat are associated with marshy areas and open forests with a species rich herb understory. Our results demonstrate that shrub reduction though cattle grazing in coastal scrub results in fewer numbers or complete loss of many shrub associated such as Wrentit, Orange-crowned Warbler, Benwick’s Wren, and Spotted Towhee. Grassland associated species such as Savannah Sparrow respond positively to grazing which may functionally maintain structural characteristics similar to early successional stages. Future monitoring in these parks should focus on factors that affect avian productivity and, ultimately, the viability of riparian bird species.