Male cardinals typically have bright red feathers that signal to potential mates that they are healthy and virile. Female cardinals have a mixture of duller shades of gray, brown and red because they do not have to impress potential mates with the bright colors and elaborate posturing observed in males.
The color difference observed between the plumage of male and female cardinals is a type of sexual dimorphism. Sexual dimorphism means that males and females exhibit different physical and behavioral traits that clearly identify each sex.
Male cardinals can be aggressive when defending their claimed territory, and they may attack other males that intrude.