I checked the link above and the classic Stokes Nature Guide to Bird Behavior this morning. Both indicate that the chirp call tends to function as a locator call for individual cardinals to keep in touch wth one another. It's described as making it easy to locate the bird.
Similar information is found here "Northern Cardinals produce a variety of different calls, with each call serving a different function. The "chip" call, having a metallic like quality, is the most frequent. It is used singularly and repeated slowly to allow cardinals to locate each other, and is repeated quickly to indicate alarm. They are also used to signal aggression, the volume and rate of calling indicating the level of the aggression. "