Throughout the breeding season, adult northern cardinals live in distinct breeding territories that measure anywhere from three to 10 acres on size. The size of these territories is dictated by the availability of food and suitable nesting sites.
Since cardinals will vigorously defend these territories during the breeding season; if your backyard is located within such an area; other cardinals are kept away. This effectively limits the numbers of cardinals that feed in your year during the spring and summer. This means the only cardinals you are apt to see at your feeders are a breeding pair of adults and their fledglings.
In fall and early winter, many cardinals form flocks that that daily wander about large territories large enough to contain adequate food for the flock. These flocks generally contain roughly equal numbers of males and females.
Interestingly, the birds that comprise such flocks also roost together at night.
By the same token, some adult pairs never leave their spring/summer territory.
Consequently, throughout winter flocks of cardinals will descend on backyards offering sunflower seeds and white millet. I especially like to watch these flocks feed late in the day. During this special time, often called the golden hour by photographers, their plumage seems to glow.
Winter cardinal flocks disband in late winter and early spring. When this occurs, pair of birds will once again stake out breeding territories and the numbers of cardinals visiting your feeders will plummet.