The beauty of this neotropical raptor was noted by early naturalists exploring the American Southwest, and its presence still attracts ornithologists and birdwatchers to the region. The uniqueness of this bird is reflected by its placement in the monotypic genus Asturina. An older common name for this species, Mexican Goshawk, suggests its accipiter-like characteristics, especially in flight. However, taxonomists have always recognized its close relation to Buteo, rather than Accipiter, based on characteristics of form and osteology. The exact relationship with members of Buteo has been debated; placed in Buteo in 1957, it was subsequently removed in 1997 (Am. Ornithol. Union American Ornithologists' Union 1957 , American Ornithologists' Union 1997 ).
The Gray Hawk is found from the U.S. Southwest to northern Argentina and Paraguay. Habitat varies from open thorn-scrub and savanna to tropical forest edges and clearings. Throughout its range, this hawk appears to eat mostly reptiles, captured with a short, darting flight to the ground or a tree trunk. In the United States and adjacent portions of Mexico, it forages predominantly in mesquite (Prosopis spp.) woodlands adjacent to forests of cottonwood (Populus spp.) and willow (Salix spp.) that occur along streams and rivers, which are used for nesting. In southern Arizona, its nests are concentrated along the streams and rivers of the Gila River watershed, and in Texas, along the Rio Grande watershed.
The distinctive calls of this species are a characteristic sound of these gallery forests throughout the breeding season, especially as pairs establish nesting territories. Its nest is a compact structure, usually located in the upper canopy of a cottonwood. After the young fledge and reptilian prey become less active with the approach of autumn, individuals migrate to more southerly climes, returning the following spring.
The Gray Hawk is not common within the United States, probably numbering fewer than 100 nesting pairs. Recent research has demonstrated, however, that this species can achieve high densities in favorable habitat. In addition, populations in Arizona and Texas appear to be stable or increasing. As long as southwestern rivers continue to support cottonwood forests and attendant mesquite woodlands, the Gray Hawk should remain a viable part of our avifauna. Primary threats to the species in the United States and Mexico include habitat loss due to human development and associated groundwater depletion.
Most studies of the Gray Hawk have focused on its taxonomic status. Johnson and Peeters ( Johnson and Peeters 1963 ), Amadon ( Amadon 1982a ), and Millsap ( Millsap 1986a ), along with many others, have argued for or against the separation of this species from Buteo . Although many books have published general descriptions of its natural history, little quantitative information is available for this species. Richard Glinski conducted some of the first in-depth studies (unpublished) of this bird in Arizona in the 1970s. Bibles ( Bibles 1999 ) followed up on these studies with an examination of the relationship between habitat quality and breeding success in Arizona.