Credible sightings of the Short-tailed Hawk (Buteo brachyurus) in southern Arizona, Sonora, and southwestern Chihuahua commenced in the 1980s and since then have become increasingly numerous throughout the sky islands of Arizona, New Mexico, Sonora, and Chihuahua. In this report we summarize previously unpublished sightings and breeding records from this region and compare aspects of the species’ breeding biology in Arizona and Florida. In 2007 we intensively monitored one successful nest in the Chiricahua Mountains of Arizona—the 1st fully confirmed nest in the western U.S.—and in 2009 we documented, but did not monitor, another active nest in the Sierra la Madera of Sonora—the 1st nest known in northwestern Mexico. Like other described nests of the species, both nests were placed near the tops of tall live trees, fully exposed to the sky. Both had broods of two nestlings, the maximum brood size known for the species. Both young edged from the 2007 nest in the Chiricahuas, matching the productivity of three other probable nestings in the same locality that were documented by sightings of pairs of juveniles in 2003, 2006, and 2010. Together with sightings of single juveniles in 2001, 2004, 2005, 2008, and 2009, these records suggest a very high level of local breeding success. The diet at the 2007 nest was predominantly small birds (83% of 137 identified prey), similar to the diet of the species elsewhere. The majority of Short-tailed Hawks observed so far in the sky islands, including all individuals we have seen (with the exception of one bird in northwestern Chihuahua), have been of the light morph.