Grey and Grey-lined Hawks, originally described as separate species, have been confirmed as deeply differentiated and accepted as full species by South American ornithological authorities.
Grey Hawk Buteo plagiatus, found from the south-western United States south to northern Costa Rica, and Grey-lined Hawk B nitidus, distributed from southern Costa Rica to Brazil, have until recently been considered as subspecies of Grey Hawk (formerly named Asturina plagiata, until incorporated into the buzzard genus Buteo by the American Ornithologists' Union in 2006).
However, Millsap et al (2012) summarised diagnostic plumage and biometric differences in all ages and sexes, as well as differences in their alarm calls, compounding a nine per cent genetic difference already published. These all coincide with a geographical divide in central Costa Rica, though the actual precise borders of the two form's distributions are the subject of some debate at present. A full illustration of plumage differences is included in the paper, along with alarm call sonograms.
The extent of the differences in vocalisation have been itemised further in a recent blogpost.
The South American Classification Committee have accepted the two as separate species already, along with a few other authorities.
Millsap, B A, Seipke, S H and Clark, W S. 2011. The Gray Hawk (Buteo nitidus) is two species. The Condor 113: 326-339.