Winter and summer falconiform populations were studied on a 16,167 km2 area in Mohave, Yavapai, Yuma and Maricopa Counties, Arizona between 15 January 1979 and 15 January 1981. Twenty-two falconiform species were observed on the study area; 13 species nested, 4 were present only in winter, 2 occurred only during migration and 3 were accidental. The American Kestrel (Falco sparverius), red-tailed hawk (Buteo jamacensis) and Cooper's Hawk (Accipiter cooperii) were the most common species in both seasons. Of 10 plant communities studied (montane conifer forest, pinyon-juniper woodland, interior chaparral, desert grassland, joshuatree-creosotebush desertscrub, paloverde-saguaro desertscrub, creosotebush-bursage desertscrub, mixed broadleaf riparian forest, cottonwood-willow riparian forest and mesquite-saltcedar woodland--see text for scientific names of plant species) highest winter falconiform abundances occurred in cottonwood-willow riparian forest, mixed broadleaf riparian forest and desert grassland communities. Highest abundances in summer were in cottonwood-willow and mixed broadleaf
riparian forests and montane conifer forests. Winter falconiform species diversity was greatest in desert grassland, interior chaparral and creosotebush-bursage plant communities and summer falconiform diversity was greatest in montane conifer forest, cottonwood-willow forest and desert grassland.
Analysis of interspecific distributional relationships indicated vegetation structure and prey base strongly influenced falconiform occurrence and abundance. Falconiform habitat use overlap also varied with diet; species which prey heavily on birds tended to overlap little in distribution, species which prey on mammals exhibited no strong distributional relationships and species which prey on invertebrates overlapped strongly in distribution. Only 2 neotropical species, the black hawk ( Buteogallus anthracinus ) and the zone-tailed hawk (Buteo albonotatus) fed heavily on lower vertebrates, and these falconiformes hunted different habitat types.
Several land use activities including livestock grazing, mining and power line installation altered vegetation structural characteristics, prey populations or habitat quality sufficiently to affect changes in falconiform populations. Several management activities are recommended to mitigate negative impacts of these and other activities.