One common idea is that cats are attempting to lure birds closer or capture their attention in the interest of predation by mimicking their vocalizations. In such a case, this behavior would amount to more than chatter, as chatter implies nonsensical sounds.
The trifecta of cats in the preceding video appear intently focused on this task, and the cat in the center at times appears fatigued. Whether this is a result of intense concentration, a different reason, or not even tiredness at all remains in the realm of speculation and fancy.
Another hypothesis purports that these vocalizations are a result of either excitement at seeing potential prey, or frustration at not being able to acquire said prey via a vocal displacement of this frustration.
Observations of wildcats in the Brazilian Amazonian jungle by field biologists do, however, lend some credence to the hypothesis of genuine mimicry. In 2005 researchers from the Wildlife Conservation Society (WCS) observed Margay wildcats emitting calls to pied tamarin monkeys.
These calls not only sounded identical to typical tamarin calls, but elicited observable recognition and confusion in the monkeys. Fabio Rohe, one of the researchers involved in wildlife conservation in the Brazilian Amazonian jungle, believes domestic felines have this copycat ability as well.
Excerpts from the 2005 WCS Margay project
So the next time you see a domestic cat (felis catus) engaged in this behavior, there's a good likelihood that what you're witnessing is not a random activity or individual cat histrionics. Rather, it could well be a cunning predation tactic informed by several million years of felid evolution!