I had a flock of cardinals in my yard all winter. There were never fewer than five males, and an unknown number of females. The most beautiful picture I didn't take was when all five males were regally resplendent against the snow covered cedar tree. I saw that shot five or six times this winter, but never managed to grab the camera in time. Maybe next year!
I thought my gorgeous flock of cardinals was unique to my backyard "sanctuary." I had only seen pairs of cardinals before - maybe two pairs at a time (and I've been birding for 40+ years!) But according to a plethora of reputable sources, Northern Cardinals do form flocks during the winter months. In fact, it is not unusual to see flocks of sixty or more!
Of course, I now know why I never saw so many cardinals before: its because I never lived in the wildlife mecca of FRAMINGHAM before!