Adapted from Jim Conrad's online book A Birding Trip through Mexico, This excerpt from "Hot & Steamy at Témoris Station" in western Chihuahua state
Today, right on cue, it appears among thick bushes next to the river. It's the Wilson's Warbler, a mostly yellow, slender little bird, incessantly working among streamside branches stretching over the water reaching for light. The bird darts beneath this leaf, then that one, flits a tiny distance, pokes its tiny, slender bill into miniscule angles between leaf petioles and the stems they arise from, habitually twitches its tail, flits a little farther or maybe does an aerial cartwheel to catch something too small to show through binoculars, suddenly hops onto side-branches to snap up spiders or mites, and it does most of this poking and bill-snapping so fleetingly that once it's over it's hard to say whether it happened at all. Frenetic activity is typical warbler foraging behavior, but the pace of the Wilson's Warbler is more lively than most.
Témoris lies a little north of the winter range the field guide describes for Wilson's Warblers. The winter range extends from central and southern Mexico all the way south through Central America to Panama. Therefore, probably the bird seen now is just resting during its continuing migration south. Moreover, since this is so late in the migrating season, it's a good bet it has come from the northern part of the species' summer range.
After some thirty years of birding in Mexico, my impression is that this species is probably the most ubiquitous, or at least the most frequently seen, wintering songbird of all of Mexico. This is curious to me since in the eastern US I seldom see it.