Yellow-rumped Warbler...Closer Sustained Look

Yellow-rumped Warbler (Dendroica coronata) hasn't arrived here in western Montana just yet. In your part of the country they may be around and building in spring numbers. Yellow-rumps are numerous in migration just about everywhere making it a good species to observe regularly and really get to know. I took a yellow-rump photo out my window (Bitterroot Valley of western Montana) three years ago during May. I came across this photo recently and recognized its value, i.e. what subspecies is this and why? I admit to being lax at examining birds as well as should be done, in this case down to subspecies. If you are a novice birder, knowing common birds well is a key behavior to advance field identification skills. Here's the photo for a simple challenge or review, enjoy :-)

Yellow-rumped Warbler