840 The Wilson Journal of Ornithology 120(4):840–855, 2008 REGIONAL ANALYSIS OF RIPARIAN BIRD SPECIES RESPONSE TO VEGETATION AND LOCAL HABITAT FEATURES NADAV NUR, 1,2 GRANT BALLARD, 1 AND GEOFFREY R. GEUPEL 1 ABSTRACT.—We investigated relationships between riparian bird abundance and local vegetationcharacter- istics and habitat features across the Sacramento/San Joaquin Valley, California. Number of detections was analyzed for each of 21 species from point count surveys over a 4-year period at 22 sites from three regions (Sacramento River, Cosumnes River, and San Joaquin River) in relation to 16 measures of habitat and vegetation composition within 50 m of 184 survey points. Tree variables, including tree height and trunk diameter, were often important, as was specific composition of tree species, especially Fremont cottonwood (Populus fremontii) and valley oak (Quercus lobata). Effects of mugwort (Artemisia douglasiana) and blackberry (Rubus spp.) were generally positive. The median partial R 2 due to vegetation/habitat characteristics was 16% after controlling for regional differences in abundance per species. Comparisons of model results at the local versus regional scale revealed spatial variation in bird abundance was independent of spatial variation in habitat variables. The effect of a habitat variable differed among the three regions for 11 of 16 variables. Models that used one or more of the first three principal components (extracted from the 16 vegetation and habitat variables) had substantially lower predictive ability than models built using individual variables. The results emphasize the importance of both understory vegetation and tree characteristics at different spatial scales. Local vegetation and habitat char- acteristics are important in explaining variation in local abundance, but there is a need to develop models specific to each subregion. Received 28 July 2006. Accepted 1 March 2008. Riparian habitat in the western United States is one of the most productive and valu- able habitats for all wildlife, especially in Cal- ifornia (Knopf 1985, Rich 2002, Faber 2003). It is also one of the most threatened habitats, with only 5% of California’s original riparian habitat remaining (Katibah 1984, Abell 1989). Riparian restoration and management of ri- parian habitat have become management pri- orities for agencies and non-governmental or- ganizations throughout California (RHJV 2004). Successful management and conservation of birds using riparian habitat requires infor- mation on how birds respond to habitat char- acteristics, including changes resulting from habitat degradation or restoration. Specific in- formation is needed regarding elements or at- tributes of habitats used by avian species. Ear- lier studies on bird-habitat associations em- phasized general structural characteristics of vegetation (Lack 1933, Hilde ´n 1965, Wiens 1969, Willson 1974, Cody 1985), but more recent studies have identified the importance of specific tree species for riparian-dependent birds (Strong and Bock 1990, Saab 1999). This is consistent with findings of Wiens and 1 PRBO Conservation Science, 3820 Cypress Drive, #11, Petaluma, CA 94954, USA. 2 Corresponding author; e-mail: nnur@prbo.org Rotenberry (1981) that shrub-steppe birds re- spond more strongly to specific shrub species than to general vegetation and habitat struc- ture. Riparian vegetation is composed of ground cover, herbs and forbs, shrubs, and trees; few studies have examined the impor- tance of all components (Heath and Ballard 2003). We also do not know the spatial scale at which birds respond. Previous riparian studies have either had broad spatial coverage (e.g., Tewksbury et al. 2002) or have examined fac- tors influencing abundance at the local, terri- tory-level scale (e.g., Strong and Bock 1990), but have rarely combined both components. Saab (1999) concluded that regional-scale fac- tors were more important than local habitat factors in explaining variation in riparian bird abundance, but this conclusion has not been supported in other studies (Scott et al. 2003). Little information is available as to whether bird-habitat correlations at a site or cluster of sites apply across larger spatial scales (e.g., across different watersheds or regions). This limits our ability to successfully generalize management recommendations based on one local study to that of other populations. Riparian bird studies have often examined community-wide metrics (i.e., species diver- sity or richness), but relatively few have an- alyzed species-specific patterns of abundance