Photo courtesy of Helen & Noel Snyder
COMMON or LESSER BLACK-HAWK (Buteogallus anthracinus) HABITAT REQUIREMENTS AND LEGAL STATUS.
- Schnell, J.H., R.L. Glinski, and H.A. Snyder. 1988. Common Black-Hawk. pp. 65-70 in R.L. Glinski, B.G. Pendleton, M.B. Moss, M.N. LeFranc, Jr., B.A. Millsap, and S.W. Hoffman. Proceedings of the Southwest Raptor Management Symposium and Workshop, Natl. Wildl. Fed., Washington, D.C.
Legal Status (p. 65)
State threatened species in Texas and Arizona. State endangered species in New Mexico. Federal protected migratory bird but not listed as federal endangered or threatened species.
Population Status and Trends
A total of 220-250 pairs in Arizona, New Mexico and Texas. Only 4-8 pairs estimated to exist in Texas. Texas may have formerly supported a greater abundance of this species along the Rio Grande. Occupies United States as a peripheral species that ranges over much of northern South America, Central America and Mexico.
Distribution (p. 65)
Peru and Guyana north through Central America to Mexico and the Southwestern United States.
Habitat (p. 66)
"obligate riparian nester, dependent on mature, relatively undisturbed habitat supported by a permanent flowing stream."
"Streams less than 30 cm deep of low to moderate gradient with many riffles, runs, pools, and scattered boulders or lapped with branches provide ideal hunting conditions".
Diet (p. 68)
In most settings in the southwestern U.S. Common Black-hawks feed predominately on fish and frogs, but also capture a wide variety of prey including insects, small and medium-sized birds, lizards, snakes, and turtles. Schnell et al. list 57 different prey taxa, including 8 lizard, 4 snake, 9 bird, and 7 mammal species. Although usually considered to be linked obligately to aquatic prey, under some conditions black-hawks feed predominately on birds and mammals.
Limiting Factors ( pp. 68-69)
"The greatest threat to the common black-hawk's survival in the United States is alteration and elimination of riparian habitat through: (1) clearing for phreatophyte control, or to create agricultural and pasture lands; (2) water diversion for irrigation and storage; (3) diking and damming for flood control; and (4) lowering of the water table through underground pumping."
Schnell et al. also implicated less acute processes such as livestock grazing and proliferation of Salt Cedar (Tamarix cinensis) as destroying habitat by blocking regeneration of riparian forests of native vegetation.
Among a list of disruptive human factors Schnell et al. also mention ". . . recreational use by hunters, hikers and campers (especially camping and swimming close to nest trees)". and " Although some black-hawks at Aravaipa have become conditioned to tolerate a high level of human disturbance, it would be wrong to conclude that these disruptions result in no adverse effects".
Management and Habitat Restoration (pp. 69-70)
"A large degree of habitat improvement can be achieved by reducing disruptive human activities and protecting the prey base. Many riparian corridors lack regeneration of gallery forest tree species".
Regenerate riparian trees. Protect tree seedlings for 3-5 years from livestock grazing. plant new seedlings. Increase prey sources by constructing small impoundments (about 17 m in diameter) to concentrate fish and frog prey.
- Schnell, J.H. 1988. Lesser Black Hawk pp. 379-389 in R.S. Palmer (ed.) Handbook of North American Birds, Vol. 4. Yale Univ. P, New Haven.
Habitat requirements and management
"The continued health of these birds depends on the preservation of relatively isolated natural riparian habitat that contains permanent flowing water (supporting aquatic and semiaquatic vertebrates) and gallery-forest tree species for nesting. A short-term management procedure to enhance their productivity would be erection of low cross dams near nest sites. These should not stop natural flow but should form impoundments large enough to concentrate frog and fish populations, important components of this hawk's diet. Long-term management objectives must include continued regeneration of gallery-forest seedlings by periodic suppression or elimination of livestock grazing" (p. 388)
ECOLOGICAL SIGNIFICANCE OF SOUTHWESTERN RIPARIAN WOODLANDS
- Ohmart, R.D. 1994. The effects of human-induced changes on the avifauna of western riparian habitats pp. 273-285 in J.R. Jehl, Jr. and N.K. Johnson (eds.) A century of avifaunal change in western North America. Cooper Ornithological Society: Studies in Avian Biology No. 15.
Note: This paper provides a good window to an extensive body of literature regarding western and southwestern riparian woodlands. Ohmart is one of the experts, and perhaps the principal expert on restoration of southwestern riparian woodlands.
Significance of Southwestern and Western Riparian Woodlands
"A minimum of 95% of the riparian habitats in the west have been lost, altered, or degraded by human-induced change" (p. 273).
Factors responsible for destruction and degradation of western riparian woodlands: ". . . water management activities (dams, reservoirs, instream flow reductions, flood control and dewatering of rivers), domestic livestock grazing and agriculture. . . recreational activities, mining, and timber harvesting. . . " (p. 273). The Arizona Nature Conservancy (1987 reference not listed in Ohmarts Literature Cited) considers the desert riparian cottonwood-willow forest as ". . . the rarest forest type in North America".
Use of Southwest and Western Riparian Woodlands by Birds
"Riparian habitats, though tiny in area, have been reported to support as many breeding pairs of birds/unit area as the best avian habitats in the United States" (p. 275).
Ohmart reviews published studies regarding use of southwestern and western cottonwood-willow riparian woodlands by birds. This review found that 43%-82% of local birds species depended at least partly on riparian habitats for essential habitat features such as nesting sites, and feeding and watering areas. He also points out that in Arizona and New Mexico 40% and 50%, respectively, of state-listed endangered species depend on aquatic or riparian habitats. This level of utilization is impressive considering that current western riparian woodlands occupy less than 1% of the total landscape.
Ohmart summarizes evidence of links between vegetation species composition and structure, and livestock grazing and bird species diversity and abundance. Cottonwood-willow woodlands with dense shrubby understories always support avian densities than woodlands dominated by the exotic species, Tamarisk (Tamarix aphylla). Exclusion of cattle from riparian forests tends to trigger increases in avian abundance.
Restoration of Southwestern riparian Cottonwood-willow forests.
Ohmart also lists the following recommendations for restoring riparian communities: (1) Plant native riparian trees and shrubs in holes augered deep enough to allow root systems to reach water tables lowered by stream channelization; (2) monitor salinity of ground and surface water; (3) plant native tree and shrub species that have the highest value to wildlife; (4) give riparian communities 8-10 years of total rest from livestock grazing; (5) restore riparian woodlands in large blocks; and (6) plant a variety of tree and shrub species to promote structural and taxonomic diversity. His past publication provide more detailed descriptions of procedures.