Avian Use of a Desert Riparian Island and Its Adjacent Scrub Habitat

-A riparian bird community in central Arizona contributed from 23 to 33% of the birds along the adjacent desert washes, and from 7 to 15% of the birds in the adjacent desert upland. Conversely, the desert bird community contributed only from 1 to 1.5% of the birds in the riparian island. Bird density ranged from 336 to 446 birds/40 ha in the riparian core and edge in 1981 and 1982. Bird density in the adjacent desert decreased with distance from the edge of the riparian island to a low of 10 1 birds/40 ha in 198 1, to 137 birds/40 ha in 1982, in the segment 600 to 1,000 m from the riparian edge. Riparian woodlands in the southwestern United States are extremely important to bird populations (Carothers et al. 1974, Stamp 1978, Ohmart and Anderson 1982. These woodlands are often considered isolated habitat islands, yet adjacent areas may be important in determining riparian bird population densities and composition (Stevens et al. 1977, Shurcliff 1980, Szaro 1980). Neighboring areas can include many different vegetation types, each with its own complement of breeding birds that are potentially capable of competitively restricting riparian birds (Carothers et al. 1974). Differences in availability of food and nest sites in adjacent habitats may further affect riparian bird density and species richness, even in the same riparian type (Goldberg et al. 1979). Conversely, riparian birds may influence bird population densities and species composition in the environs (Conine et al. 1978, Hehnke and Stone 1978, Wegner and Merriam 1979). We sought to determine the extent to which the bird communities of a riparian island and the surrounding desert scrub influence each other. We examined several questions concerning the interaction between the two communities: (1) which bird species, if any, are found exclusively in either habitat? (2) what is the contribution, in terms of densities and species richness, of the bird populations in either habitat to each other? (3) how far do riparian or desert birds venture into other habitats? and (4) are there any differences in summer versus permanent resident use of the habitats? Answers to these questions will advance our understanding of the role that riparian habitat islands play in determining bird community structure in the arid southwestern United States. STUDY AREA We conducted our study on the Tonto National Forest on Queen Creek, about 3.7 km upstream from the mouth of Whitlow Canyon and about 16 km west of Superior, Arizona. The area consists of a 15-ha stand of Goodding willow (Salix gooddingii) and salt cedar (Tamarix pentandra), surrounded by Sonoran Desert scrub (Szaro and DeBano 1985). The riparian stand is rectangular, approximately 350 m by 450 m. A border of vegetation approximately 35 m wide along the edge of the stand is composed of salt cedar, seep willow (Baccharis glutinosa), and velvet mesquite (Prosopis julzjlora). Two washes extend northward, perpendicular to the long edge of the stand. Their vegetation primarily consists of catclaw (Acacia greggii), velvet mesquite, desert hackberry (Celtis pallida), creosote bush (Larrea tridentata), seep willow, and foothill palo Verde (Cercidium microphyllum). On the upland benches, which extend from the riparian area parallel to the washes, the vegetation is foothill palo Verde, saguaro cactus (Cereus giganteus), jumping cholla (Opuntiafulgida), jojoba (Simmondsia chinensis), Engelmann prickly pear (0. phaeacantha), creosote bush, teddy bear cholla (0. bigelovii), and ocotillo (Foquiera splendens). For a more complete description of the vegetation, see Szaro and DeBano (198 5).