I now know what they are! They are not warblers after all! They are Western Kingbirds!
I emailed the head of the birding group for Utah, and he identified them.
At least I've been feeding them the right foods, as they are bug-eaters, not seed-eaters.
I could not believe the size of treats she brought yesterday! A big ole grasshopper (which got away as I was trying to assist with tweezers) and a giant dragonfly, which Donna was able to choke down.
I swear those bugs are nearly big enough to swallow the birdies!
I must leave for work shortly, so it will be up to Mama Kingbird (Queenbird?) to take care of them. The window is open and I put a curtain rod down by their perches so she can easily reach them.