Ferruginous Hawk is the largest member of the genus Buteo in North America. It is an uncommon grassland specialist which has been declining in recent years and listed as threatened in Canada. It is a winter visitor to California where it occurs in two distinct color morphs (light and dark) with dark morphs, such as this being rare. Dark morphs comprise fewer than 10% of birds breeding in Alberta and fewer than 5% elsewhere. I believe this individual is probably an adult based on molting secondaries.
Canon PowerShot SX60 HS.
References:
Ng, J., M. D. Giovanni, M. J. Bechard, J. K. Schmutz, and P. Pyle (2017). Ferruginous Hawk (Buteo regalis), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.ferhaw.02
Schmutz, J. K. and S. M. Schmutz. (1981). Inheritance of color phases of Ferruginous Hawks. Condor no. 83:187-189.