Western Kingbird was eliminated because of the thick bill, darker gray coloration, brown tail and lack of white edging, and ultimately vocalizations. A comparison shot with both Tropical and Western Kingbirds in the same frame were obtained later. 

Cassin's Kingbird was eliminated because of the thick bill, yellow/green breast, and vocalizations. 

Thick-billed Kingbird was eliminated because of the lack of a black cap, less white in the throat, smaller bill, and black wings. 

Couch's Kingbird looks identical to Tropical and can apparently only be identified without hesitation in the hand. The vocalizations clinched the ID of Tropical. 

We learned later that the vocalizations we heard are called by Nathan Pieplow as Chitter calls.

I have seen Tropical Kingbird in Mexico before but it was a couple of years ago. I have never seen a Couch's Kingbird before. 

I am very familiar with Western and Cassin's Kingbirds and that familiarity helped me to notice some differences initially. 

Jason was much more familier with the ID between Couch's and Tropical Kingbirds and knew that we needed vocalizations to separate them. Would not have been able to come to this ID without his knowledge.