The one on the left is about the size of a canary - and may be a canary. (After consulting Ridgely and Gwynne's A Guide to the Birds of Panama, 2nd edition, this appears to be an immature migratory Yellow Warbler.)
The one on the right is somewhat larger (not apparent in this picture because he is farther away). He has a gray breast, and brown flashing around his neck, striped wings and a distinctive eye stripe and head tuft. He reminds me of a sparrow on steriods. (This one was confirmed to be a Roufous-Collared Sparrow.)
Here is a close-up of him:
The second one was agressive and ran the yellow one away. Could this be the origin of the slang word "yellow" (meaning coward)?
Later I saw two more new birds in the guayaba tree. One was a very large black hummingbird that could be a Green-Breasted Mango:
The other could be Big Bird's cousin, most closely resembling a female White-Winged Tanager: