Odd kingbirds photos

During a visit to the Metropolitan Natural Park (PNM, in Panama City) last tuesday, agust 10th, I found a flock of more or less 20 Tyrannus Kingbirds. I took only four (4) photos, showing three different individuals. All the three birds in the photos show pale heads (specially the throat) with contrasting dark "mask" (including the last photo of a semi-concealed bird), and yellow underparts including the chest, which have little -if any- olive on it. Also, some photos show pale edges in the coverts, notched tails and smallish bills. These marks suggest White-throated Kingbirds, Tyrannus albogularis (never reported out of South America, but migratory in its amazonian distribution), instead of the expected Tropical Kingbirds. Two of these photos were published in XENORNIS and then Ken Allaire kindly posted to ID Frontiers on my behalf with part of the text that I e-mailed him:

"Last tuesday I saw this flock of kingbirds at the highest lookout of the PNM [135 meters above sea level]. In the field I thought it was weird to see all these TK's [Tropical Kingbirds] together (around 20) and all seemed to be quite small, resembling a flock of Eastern Kingbirds. They were eating lots of fruits, but also flycatching. The only reason why I took the photos was precisely that they looked "pretty", you know, with a nice yellow colour of the underparts, including the chest and not like usual TK's (with olive wash to the chest and gray heads). They did not vocalized."

I'm now showing all the original pictures, each one followed by one or two cropped versions. I did not change colours, contrast or sharpness. Feel free to comment about the identity of these birds or follow the discussion at ID Frontiers.