Both sexes of the Northern Cardinal sing. Their song is a loud, clear two-part whistle sometimes speeding up and then slowing down again towards the end. The song is typically 2 or 3 seconds long and repeated over and over, of course. It sometimes sounds, to me, like they are signing birdie, birdie, birdie. The peak of their singing is in typically spring to early summer, though the males esp. may sing all year long.