COMPARATIVE USE OF RIPARIAN CORRIDORS AND GASES BY MIGRATING BIRDS INSOUTHEAST ARIZONA

The relative importance of cottonwood-willow riparian corridors and is olated oases to land birds migrating across southeastern Arizona was e valuated during four spring migrations, 1989 to 1994, based on pattern s of species richness, relative abundance, density, and body condition of birds. We surveyed birds in 13 study sites ranging in size and con nectivity from small isolated patches to extensive riparian forest, sa mpled vegetation and insects, and captured birds in mistnets. The cont inuous band of riparian vegetation along the San Pedro River does not appear to be functioning as a corridor for many migrating species, alt hough it may for a few, namely Yellow-breasted Chats (Icteria virens), Summer Tanagers (Piranga rubra), and Northern Rough-winged Swallows ( Steldigopteryx serripennis), which account for fewer than 10% of the i ndividuals migrating through the area. Small, isolated oases hosted mo re avian species than the corridor sites, and the relative abundances of most migrating birds did not differ between sites relative to size- connectivity. There were few differences in between-year variability i n the relative abundances of migrating birds between corridor and oasi s sites. Between-year variability decreased with overall abundance of species and was greater for species with breeding ranges that centered north of 50 degrees N latitude. Body condition of birds did not diffe r relative to the size-connectivity of the capture site, but individua ls of species with more northerly breeding ranges had more body fat th an species that breed nearby. Peak migration densities of several bird species far exceeded breeding densities reported for the San Pedro Ri ver, suggesting that large components of these species were en route m igrants. Peak densities of Yellow Warblers (Dendroica petechia) reache d 48.0 birds, ha, of Wilson's Warblers (Wilsonia pusilla) 33.7 birds/h a, and of Yellow-rumped Warblers (D. coronata) 30.1 birds/ha. Riparian vegetation is limited in extent in the vicinity of our study sites, c overing less than 1% of the landscape. We conclude that all riparian p atches in southeastern Arizona are important as stopover sites to en r oute migrants regardless of their size and degree of isolation or conn ectivity. In light of potential habitat limitation, the protection of both small, disjunct riparian patches and extensive riverine tracts in western landscapes is imperative.