Use of Tree Species by Forest Birds during Winter and Summer

Abstract

We recorded the foraging behavior of bird species during winter and summer in a mixed conifer forest of the western Sierra Nevada. All bird species increased their relative use of incense cedar (Calocedrus decurrens) in winter compared to summer. We noted no obvious difference in the vigor (health) of trees used by birds between seasons. Most species revealed a general trend towards increased use of the bark of incense cedar in winter relative to summer. It appears that birds are able to obtain food from under the loose, flaky bark of incense cedar more easily than from under the more firm, compact bark of other timber species in winter. The overwinter survival of birds may be lowered by reduction in the stocking level of small (<30 cm dbh) incense cedar. J. WILDL. MANAGE. 49(4):1098-1102 Birds are known to alter patterns of habitat use and foraging behavior between seasons (Willson 1970, 1971; Travis 1977; Conner 1980, 1981; Hutto 1981; Lewke 1982). These variations are of importance in land use planning because the manager may have to allow for a different suite of habitat needs on both speciesspecific and season-specific bases. Although vegetation structure (both vertical and horizontal) has been inferred to be important in influencing the species composition and abundance of birds (MacArthur and MacArthur 1961, Karr and Roth 1971, Willson 1974, Roth 1976), bird diversity may also be influenced by the diversity of plant species present, independent of structural considerations (MacArthur and MacArthur 1961, Holmes and Robinson 1981, Robinson and Holmes 1984). Therefore, the task of providing adequate habitat is complicated by many factors. The land manager changes the composition of plant species in an area based on numerous, often competing, factors (e.g., economical, biological, sociological). In the western Sierra Nevada, for example, the composition of the forest i being changed as areas are harvested and replanted (converted) to economically desirable species. Although the response of birds to gross cha es in their habitats has been examined (e.g., see Verner and Boss 1980), the effects of more subtle changes in plant species composition are not well known, especially interseasonally. Our objective in this study was to determine if the use of tree species by birds differed between winter and summer in the western Sierra Neva a. Our intent was to describe general patterns that might deserve increased attention by