In most North American songbirds, the males are the only ones putting on a vocal show. Males sing to attract mates and to establish territory. Female Northern Cardinals are exception to this rule. They are among the few female songbirds that sing, too. The "whisper song" in this audio clip features a pair of Northern Cardinals perched close together.
Next time you hear a Northern Cardinal singing - don't assume you're looking for a bright red male. It could be a female! To learn more about Northern Cardinals, visit Cornell's All About Birds.