I'm not 100% positive on this, but I'm leaning towards Tropical Kingbird over Couch's Kingbird due to the greenish color on the back. (Tyrannus melancholicus)
Unfortunately, the same can be said about all the kingbird photos. Steve Howell perhaps said it best in his field guide to Mexican birds: "Plumage differences [between tropical and couch's kingbird] are at best subtle and not practical for field use." There are differences in back color, beak size, etc, but only on average with plenty of overlap between the two species.
