Those chirps are calls as opposed to the loud and varied songs which both the male and female cardinal sing. Cardinals are an exception in North America because of this female song-singing ability. Why birds sing. As a general rule, birds sing in connection with sexual reproduction. Biologists over the years have found that birds sing to stake out and defend territories, to identify themselves, to attract the opposite sex, to stimulate nest- building and mating, and to encourage the female to incubate her eggs.
Some studies suggest that sometimes, male birds sing simply because they're happy or enjoy singing.
There is strong evidence that baby birds hear their species' song while still inside their shell. The song is imprinted on their brain circuitry at this time. They are able to produce a rudimentary song, but are unable to produce the species' adult song. Birds have accents. While it's easy enough to identify a cardinal song in Key West or in Indiana, there is marked variation in the details of those songs.