Cooper’s hawks are fairly variable in size. There is usually minimal to no overlap in dimensions between the sexes, with females being considerably larger than males. On average, she may be about 20% larger linearly and around 40% heavier (though can be up to 125% more massive). More westerly Cooper’s hawks (roughly west of the Rocky mountains) show slightly less pronounced sexual dimorphism than hawks of the species elsewhere. Sexual dimorphism in Cooper’s hawks is most reliably measured by wing size, talon size, then body mass. Although there is some margin of error, within a given region dimensions of the two sexes never overlap in these regards (but may overlap marginally in tarsal and tail lengths). In general terms, Accipiter species are among the most sexually dimorphic in size of all raptorial birds.
An eastern population ranges west through southern Canada from southern New Brunswick and Ontario to the eastern edge of the U.S. Great Plains, south to Florida, the Gulf Coast, and eastern Mexico. Only northernmost populations are migratory. A western population breeds west of the Sierra Nevada from northern California to northern Baja California, and has recently expanded into Oregon and Arizona, and east of the Sierra Mountains in California.