The San Pedro River serves as a corridor between the Sky Islands of the Madrean Archipelago in northern Sonora and southern Arizona in its southernmost reaches and, in the north, Arizona?s Central Highlands. The river represents a ribbon of water and riparian vegetation in an otherwise arid environment. The river exhibits a remarkably high biodiversity, both in resident and migratory species. The avian species of note that qualify the site as an IBA are Southwest Willow Flycatcher, a federal endangered species; Yellow-billed Cuckoo; Northern Beardless Tyrannulet, Tropical Kingbird, Bell?s Vireo, Lucy?s Warbler.Yellow Warbler (sonoriensis), Gray Hawk, Common Black Hawk, Zone-tailed Hawk and the majority of nesting Mississippi Kite in Arizona. The 59 miles of the Lower San Pedro River from the Narrows north of the community of Cascabel to the Gila River confluence are surrounded by saguaro cactus-dominated Sonoran Desertscrub. This reach of the river is characterized by well developed cottonwood-willow gallery forest riparian habitat interspersed with well developed mesquite (Prosopis juliflora) woodlands known as bosques and includes the largest intact mesquite bosque community in Arizona on 14 miles of the San Pedro River beginning south of the community of San Manuel and ending north of the community of Mammoth. The majority of the land is privately owned. About 22 miles of river properties are in public ownership, managed by The Nature Conservancy or are under conservation easement or conservation management. These owners have agreed to be acknowledged as partners in the IBA.
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Species of raptors that nest on the lower San Pedro River include Gray Hawk (Asturina nititda=Buteo nitidus), Mississippi Kite (Ictinia mississippiensis), Common Black Hawk (Buteogallus anthracinus), and Zone-tailed Hawk (Buteo albonotatus). Western Yellow-billed Cuckoos (Coccyzus americanus occidentalis), currently a candidate for Federal listing as a threatened or endangered species, nest in numbers on the lower reaches of San Pedro River.
The high importance of the lower San Pedro River for the recovery of the Southwestern Willow Flycatcher contributed to its designation as critical habitat for the species. Critical habitat includes approximately 60 river miles of the lower San Pedro River between points approximately 3.5 river miles south of Hot Springs Canyon to the Gila River confluence. Arizona Game and Fish Department Surveys documented within the Winkleman Study Area 143 pairs in 2004 representing 43.8% of the total Arizona monitored nests (Source 1A) and 136 pairs in 2005 representing 43.45% of the total Arizona monitored nests (Source 1B). These data and the 157 pairs recorded at locations in the nomination area in 2004 and qualify the nomination for B1 Continental designation for this species. (Thresholds are 20 breeding pairs/60 individuals for Empidonax traillii extimus).
The reach between Three Links Farm, which is in a conservation easement held by The Nature Conservancy and included in the IBA nomination package, and the Gila River confluence is densely occupied by Southwestern Willow Flycatchers at sites where there is suitable habitat. In 2005 the reach thus described contained 164 southwestern willow flycatcher territories consisting of 308 adult birds. Over 40% of the nesting population in Arizona is found here.
The mesquite bosque at 7 B Ranch supports excellent breeding populations of Lucy?s Warbler and Bell?s Vireo. Gavin and Sowls documented bird diversity at this same location through repeated monitoring during a nine month period in 1974 and 1975.
Two survey efforts were organized by the Audubon Society in the spring and summer of 2006. Volunteer surveyors completed 9 1 kilometer long transects and 18 point counts positioned at the beginning and end of each transect on May 20, 2006. Conservation species total numbers observed were as follows: Abert?s Towhee-50; Bell?s Vireo-88; Broad-billed Hummingbird-2; Gray Hawk-5; Gilded Flicker-1; Hooded Oriole-15; Lucy?s Warbler-91; Northern Beardless-Tyrannulet-5; Olive-sided Flycatcher-1; SW Willow Flycatcher-12; Summer Tanager-46; Yellow Warbler-186.
The second survey effort occurred just before the monsoon rains and five of the nine transects and ten of the eighteen point count locations were surveyed on July 25, 2006 (4 transects: Aravaipa Creek confluence, Dudleyville crossing, Cook?s Lake, TNC Preserve north of Dudleyville) and August 2, 2006 (San Manuel Crossing). Flood conditions prevented access to the 3 Links Farms transects and scheduling conflicts resulted in the 7B transect not being run a second time.
The transect and point count surveys, CBC, and the NAMC do not adequately detect the raptors that nest on the lower San Pedro River. Gray Hawk, Zone-tailed Hawk, Common Black Hawk, and Mississippi Kite are represented on the entire reach. Levy (1971) documented up to 10 Mississippi Kite nests and Corman and Gervais-Wise (2005) describe the lower San Pedro River as being the Arizona breeding population stronghold and supporting the majority of nesting sites for this species in Arizona. Corman and Gervais-Wise describe the Gray Hawk as ?locally common? along segments of the San Pedro River. The six Gray Hawks observed on the May 20, 2006 transect surveys supports this claim. An organized survey for raptors is recommended to better catalog the densities and occurrences.
Arnold, L.W. 1940. An ecological study of the vertebrate animals of the mesquite forest. University of Arizona. M.S. Thesis.
English, H.C., A.E. Graber, S.D. Stump, H.E. Telle, and L.A. Ellis. 2006. Southwestern Willow Flycatcher 2005 Survey and Nest Monitoring Report. Technical Report 248. Research Branch, Wildlife Management Division, Arizona Game and Fish Department, Phoenix, Arizona. 82 pp.
Gavin, T.A. and L.K. Sowls. 1975. Avian Fauna of a San Pedro Valley Mesquite Forest. Journal of the Arizona Academy of Science. 10(1):33-41
Groundwater extraction, improper livestock grazing in riparian corridor, fire, off highway vehicles, suburban development and wood cutting are the greatest threats to this IBA. Recent threats have been a proposed interstate highway and a large subdivision near San Manuel.
The majority of land is in private ownership and includes mining companies, large ranch and farm holdings as well as smaller properties within communities that are adjacent to the river. The nature Conservancy has successfully implemented numerous conservation easements in partnership with BLM, Bureau of reclamation and Salt River Project. BLM lands are scattered and co-mingled with private lands.
The Lower San Pedro River is characterized by well developed cottonwood-willow gallery forest riparian habitat interspersed with old growth mesquite (Prosopis juliflora woodlands known as bosques. The largest intact mesquite bosque community in Arizona is located on 14 miles of the San Pedro River beginning south of the community of San Manuel and ending north of the community of Mammoth. Seven river miles of this bosque are on lands acquired by Resolution Copper Company for conservation purposes and are included in the nomination for IBA. Saguaro (Cereus gigantea), Foothill and Blue Palo Verde (Cercidium microphyllum and C. floridum), Ocotillo (Foquieria splendens), and a variety of cacti and small shrubs cover the uplands. Mesquite), Catclaw Acacia (Acacia greggii), Burrobush (Hymenoclea monogyra), and Desertbroom (Baccharis sarothroides) line xeric washes, while Goodding Willow (Salix gooddingii), Fremont Cottonwood (Populus fremontii), Velvet Ash (Fraxinus velutina), and Netleaf Hackberry (Celtis reticulata) cluster along wetter drainage ways. Interspersions of Sonoran Desert grassland typified by grama grasses (Boutaloua spp.), Three-awns (Aristida spp.), and Mulenberghia spp.