From allaboutbirds.org
The brilliant red of a male Northern Cardinal calls attention to itself when males are around. You can also find cardinals by getting a sense of the warm, red-tinged brown of females – a pattern you can learn to identify in flight. Away from backyards, cardinals are still common but inconspicuous owing to their affinity for dense tangles. Listen for their piercing chip notes to find where they are hiding.
Many people are perplexed each spring by the sight of a cardinal attacking its reflection in a window, car mirror, or shiny bumper. Both males and females do this, and most often in spring and early summer when they are obsessed with defending their territory against any intruders. Birds may spend hours fighting these intruders without giving up. A few weeks later, as levels of aggressive hormones subside, these attacks should end (though one female kept up this behavior every day or so for six months without stopping).