In fact, Couch's kingbird was considered conspecific with the tropical kingbird until 1979. The easiest way to distinguish between the two species is to listen for their call: Couch's kingbird has a raspier, more complex call that is more varied in pitch.
Couch's kingbird was previously considered to be the same species as the tropical kingbird until 1979. The most effective way to differentiate between the two is by listening to their calls: Couch's kingbird has a harsher, more intricate call that varies in pitch.