Gray hawks (Asturina nitida) range from the Amazon Basin in South America into the southwestern United States. They are migratory in the northern part of their range, arriving in southern Arizona and extreme south Texas in the spring to breed, and occasionally entering New Mexico. These northern members of the species generally depart in mid-October to overwinter in Mexico, although they can be found in south Texas year-round, and rare records exist of winter residents in Arizona. Further south in their range, gray hawks are non-migratory (Glinski, 1998; Kaufman, 2000; Stiles and Skutch, 1989; Terres, 1980).
Throughout their range, gray hawks inhabit woodlands and arid deciduous forests. In the tropics they prefer dry second growth forest and thorn scrub. They tend to select patchy open forest, forest edges, and savanna trees. It is not uncommon to find them on agricultural fields. In denser woodland they tend to keep high in the forest canopy. Gray hawk northern breeding range is found in deciduous cottonwood-willow forests and mesquite bosques along riparian corridors or in evergreen oak woodlands (Glinski, 1998; Stiles and Skutch, 1989).
Gray hawks are medium-sized, woodland buteos, with shorter wings and longer tails than typical buteos. Adult birds have a slate-gray back, finely barred gray and white underparts, a black tail with two or three white bands, and a white rump. Juveniles have dark brown backs and buff-streaked underparts, brownish-gray tails with five to nine narrow black bars, and a dark brown eye stripe. Both adult and immature birds have dark gray or black beaks, brown irises, and yellow ceres and legs. Males are smaller than females. Gray hawks fly with accipiter-like movements, alternately flapping and gliding gracefully. Their flight pattern and gray color is similar to that of northern goshawks, which led to their other common name, “Mexican goshawks.” Northern goshawks, however, have a white eyebrow and lack the tail barring distinctive of gray hawks (Glinski, 1998; Kaufman, 2000; Stiles and Skutch, 1989; Terres, 1980; Wheeler and Clark, 1995).
Males court females with calls, undulating flight, and nest building.
Gray hawks from northern populations arrive in Arizona around mid-March to breed. Nest site fidelity is high, and gray hawk territories remain fairly constant from year to year. Pairs begin building the nest right after courtship, but they partition the work. The male builds the foundation, and the female shapes most of the bowl out of green, leafy twigs from the nest tree or neighboring trees. Northerly-breeding gray hawks primarily nest in cottonwood trees, but they will also nest in willow, ash, oak, hackberry, and mesquites. They choose nest sites in the upper third of the tree, usually in branches away from the trunk. Nests are crow-sized, about 60 cm across. The female usually lays two eggs in early May, although studies of nest productivity in Arizona have recorded nests with four or even eight eggs per clutch with a mean of 1.2 or 1.1 young per occupied breeding site. The eggs are white to pale blue and rarely marked. Only females incubate the eggs. Incubation lasts about 33 days, during which the male captures food for the female. After hatching, the young stay in the nest for about six weeks.
Arizona is the northernmost stronghold of breeding gray hawks, with nearly 80 known nest sites, most of which are protected in nature preserves or conservation easements. Six pairs also nest regularly along the Rio Grande River in south Texas. There is one reported incidence of nesting in New Mexico. Further south in their range, Gray Hawks are non-migratory and nest from December through May in tall, evergreen trees (Bibles, 1999; Brandt, 1951; Glinski, 1988; Glinski, 1998; Hubbard, 1974; Stiles and Skutch, 1989; Terres, 1980).
Females alone provide incubation, but the male feeds her. The male provides most of the food for the first two weeks post-hatching, after which the female begins to hunt as well. It is not known how quickly the young can hunt for themselves once they leave the nest.
Gray hawks are swift, agile fliers that can actively pursue prey by maneuvering through trees. They perch in the forest understory, locate prey in the trees or on the ground with their keen eyesight, and make short dashes to capture it whit their talons. They take reptiles, small mammals, birds, and some insects. In Arizona, Glinski (1988) studied food types delivered to nestlings and found that the diet was composed predominantly of terrestrial and arboreal lizards (74%), garter snakes (5%), nestling and adult birds (11%), and mammals (10%). Bibles’(1999) study of nest productivity in Arizona revealed similar findings, with reptiles comprising 68.6% of prey delivered to nestlings (all were lizards but for one snake), mammals 19.6% (rodents and one rabbit), birds 9.8%, and amphibians 2% (a toad).
Bibles (1999) also revealed that gray hawks prefer home ranges with taller trees and more open understory, probably because these enable them to observe their cryptic prey more easily. The increased flight space may also facilitate greater capture success. Mesquite bosques are the primary foraging areas of gray hawks breeding in Arizona because they have these characteristics. The amount of mesquite bosque seems to be the main factor that determines habitat quality in gray hawk nesting range in Arizona. (Amadon and Phillips, 1939; Bibles, 1999; Glinski, 1988; Glinski and Millsap, 1987; Gurrola-Hidalgo and Chavez, 1996; Stensrude, 1965).
There are no documented cases of predation of gray hawks, but like other mid-sized hawks, they may be vulnerable to larger birds of prey. Raptor nestlings in the tropics may on rare occasions succumb to predation from arboreal hunters such as monkeys, coatis, snakes, and other birds, so it is possible that gray hawk nestlings may as well (Emmons and Feer, 1997).
Gray hawks are important predators of smaller lizards and snakes throughout their range, though cryptic coloration, speed, and good hiding places can help some reptile prey evade capture. Gray hawks occasionally prey upon small birds. Smaller birds will often deter predation by mobbing birds of prey to drive them away. Olive-throated Parakeets have been seen mobbing a Gray Hawk to force it to leave the area (Eitniear et al., 1990).
Since the range of gray hawks in the United States is so limited, many birdwatchers come to Arizona and Texas to see them. Bird watching has become increasingly important to the economy of southeastern Arizona. Just as the gray hawk's distribution barely extends into southern Arizona, so do the ranges of many other tropical and subtropical bird species. Several migratory species also use the area’s river corridors as they pass north from Mexico. Over 400 bird species draw birdwatchers from around the world. Their visitation has been an economic boon to the area. In 2001, visitors to two well-known birding spots in southeastern Arizona - Ramsey Canyon Preserve and the San Pedro Riparian National Conservation Area - spent an estimated $10.1 million to $16.9 million. About a third of the gray hawk nest sites in Arizona have been identified in the San Pedro NCR. While gray hawks are not the sole draw to the area, the numbers clearly illustrate the impact that conservation has on regional economies. Gray hawks, as well as people, stand to benefit from this perceived economic value. (Bibles, 1999; Relly, 2002; Viers, 2000).
Gray hawks have no special conservation status, although their limited numbers at the northern extreme of their range have led management agencies in Arizona to regard them as “sensitive species” and in Texas to consider them “threatened.” Arizona breeding populations seem to be holding steady, with reproduction balancing the losses from mortality. Fortunately, most of the 80 nest sites in Arizona today are located on protected lands. Concerned about how human demand for ground and surface water has seriously reduced desert riparian areas in the southwestern United States, private landowners and public and private conservation organizations have worked to set aside the remaining riparian corridors in southern Arizona. Further protection of the cottonwood-willow forests and mesquite bosques they support will demand protection of the waters that sustain them. If land managers succeed, this will be good news for gray hawks and the myriad other species that use these areas. Their only threat then would be recreational disturbance. Nesting gray hawks are sensitive to human activity near their nests, and many of the protected areas are frequented by recreational groups. Something as innocuous as a family picnic unknowingly staged near a nest tree can cause gray hawks to abandon their nest.
On their wintering range, other factors could lead to gray hawk decline. Glinski banded seven nestlings in Arizona that were recovered in northern Sinaloa state in Mexico, and six of these had been shot. The greatest threat on these wintering grounds, however, is habitat loss. There, the gray hawks’ thornscrub habitat is being widely cleared for agriculture. Gray hawks continue to persist along the living fence rows of trees that divide the agricultural fields, but it is unclear how many hawks this disrupted habitat can support, nor, ultimately, what effects this might have on Arizona’s breeding population. Little is known about demographics or conservation throughout the rest of the gray hawk's range (Glinski, 1988, 1998; Nabhan and Sheridan, 1977; Tellman et al., 1997; urls below).
Apparently some gray hawk pairs stay together after the breeding season is over. The gray hawks that overwintered in Arizona were a pair. Glinski called them out by mimicking their territorial breeding calls, and they responded in kind (Glinski, 1998).
Robin Kropp (author), University of Arizona, Jorge Schondube (editor), University of Arizona.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
uses sound to communicate
living in landscapes dominated by human agriculture.
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
parental care is carried out by males
Having one mate at a time.
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
scrub forests develop in areas that experience dry seasons.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
"Texas Parks and Wildlife, Texas Threatened and Endangered Birds" (On-line). Accessed April 12, 2002 at http://www.tpwd.state.tx.us/nature/endang/birds/#raptors.
"USDA Forest Service, Coronado National Forest, Santa Catalina Ranger District, Tucson, Arizona, USA. Threatened, Endangered, and Sensitive Species" (On-line). Accessed April 12, 2002 at http://www.fs.fed.us/r3/coronado/scrd/nathist/nature/tes.htm.
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Brandt, H. 1951. Arizona and its bird life. Cleveland, Ohio: The Bird Res. Found..
Eitniear, J., S. McGehee, W. Waddell. 1990. Gray hawk mobbed by olive-throated parakeets. Bird behavior, 8(2): 114-115.
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Glinski, R. 1998. The raptors of Arizona. Tucson, Arizona: University of Arizona Press.
Glinski, R. 1988. Gray Hawk. Pp. 83-86 in R Glinski, B Pendleton, M Moss, M LeFranc, Jr., B Millsap, eds. Proceedings of the southwest raptor management symposium and workshop. Washington, D.C.: National Wildlife Federation.
Glinski, R., B. Millsap. 1987. Status of the Sonora Gray Hawk Buteo nitidus maximus (van Rossem 1930). Albuquerque, New Mexico: Unpublished Report to Office of Endangered Species, U.S. Fish and Wildlife Service.
Gurrola-Hidalgo, M., N. Chavez. 1996. Serpentes: Lampropeltis triangulum nelsoni (milk snake). Predation.. Herpetological Review, 27(2): 83.
Kaufman, K. 2000. Birds of North America, a new focus on the field. New York: Houghton Mifflin Co..
Nabhan, G., T. Sheridan. 1977. Living fencerows of the Rio San Miguel, Sonora, Mexico: traditional technology for floodplain management. Human Ecol., 5: 97-111.
Relly, J. 2002. "Arizona Daily Star: Birding's big bucks. February 7, 2002" (On-line). Accessed April 12, 2002 at udallcenter.arizona.edu/publications/spncfebruary4.htm.
Stensrude, C. 1965. Observations on a pair of Gray Hawks in southern Arizona. Condor, 67: 319-321.
Stiles, F., A. Skutch. 1989. A Guide to the Birds of Costa Rica. Ithaca: Cornell University Press.
Tellman, B., R. Yarde, M. Wallace. 1997. Arizona’s changing rivers: how people have affected the rivers. Water Resources Research Center Issue Paper 19. Tucson, Arizona: University of Arizona.
Terres, J. 1980. The Audubon society encyclopedia of North American birds. New York: Alfred A. Knopf.
Viers, J. 2000. "Economic Development Institute 2000 Abstracts - SOUTHEASTERN ARIZONA BIRDNG TRAIL" (On-line). Accessed April 12, 2002 at http://tel.occe.ou.edu/edi/resource/abstract/2000abstracts/viers_j.html.
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