[Firstly, the editor apologizes profusely for sitting on this record, which sat forgotten in my inbox for two months. Now back to our programming.]
I believe it was an adult male.
The bird had a black throat, which I believe indicates a male, white underparts with no streaks, a yellow face with a slight indistinct dusky postocular mark. When first seen the evening of Jan 3 I could not note the back color. The next morning it popped up in the hillside scrub, later moving to a nearby pine, where better views were obtained.The back was gray with some dark streaking, and there was some black spotting on the crown. There were 2 white wing bars, and the in flight there were notable white flashes in the tail.
The vent did not have the yellow wash typical of Black-throated Green Warbler. Both Black-throated Green and Townsend's warblers were ruled out based on face pattern - yellow mostly unmarked face - and lack of streaking on the underparts. Golden-cheeked was ruled out by the lack of a black eyeline and again the unstreaked underparts.
The morning of Jan 4 the bird was observed for about 5-10 minutes both at eye level in hillside scrub, and in lower branches of a nearby pine, and for about 5 minutes the evening of Jan 3, when it stayed in the pines.
Craig Bennett and I were along a stream on their property when I heard the distinctive song of Northern Parula warbler in the treetops. It was the song that is ascending, ending with an abrupt note. There is an alternate Parula song with several repeated "rolling" ascending trills, but that song was not heard. The bird was heard singing for about 5 minutes. I had the Sibley call on my smartphone and played it, and it was an exact match. After about 5 minutes it stopped singing. I played Tropical Parula and the song was very different. I am not very familiar with Tropical Parula vocalizations, although I've seen many in many countries, but the recordings on the Panama bird app were quite distinct from Northern Parula.