Habitat relationships among riparian forest birds in the Eel River Delta, California

Habitat use by riparian forest birds was analyzed according to habitat types, vegetation structure and composition, and seasonal differences. Circular (0.04 hectare) vegetation plots were classified into three habitat types and contrasted along discriminant function gradients, primarily according to tree densities and shrub level characteristics. I conducted variable circular plot censuses of bird species from October 1982 to September 1983. Two models of density estimation using variable circular plot data were compared. I analyzed patterns of habitat use using a geometric method based on bird species densities and available habitat. The availability of suitable habitat and the relative importance of multivariate habitat gradients were delineated by size and shape differences in the model. Most bird species densities were greater in mature cottonwood-alder (Populus balsamifera ssp. trichocarpa-Alnus rubra) forest with an open canopy, dense understory of willows, and dense shrub-level foliage, than in other, apparently younger stands in all seasons. A few species concentrated on younger forests in one or more seasons. Many species selected uncommon subsets of cottonwood-alder forest, characterized by extremes in age-related features. Patterns of habitat use by riparian forest birds in the Eel River delta and the Central Valley of California are compared, and seasonal changes in habitat selection and use are discussed.