Aren't they the cutest little fluff bundles you ever did see? A whole flock of these came through here today, likely on their way to their breeding grounds, they are the subspecies 'Myrtle' of the Yellow Rumped Warbler (Dendroica coronatata). The western version which we see in the west, has a yellow throat but 'Myrtle' has white. The birds look smaller than what the field guide lists them as at 5 1/2".
From the Field Guide to the Birds of North America:
yellow rump, yellow patch on side, yellow crown patch, and white tail patches. In northern and eastern birds ("Myrtle Warbler"), note white eyebrow, white throat and sides of neck, and contrasting cheek patch. Western birds ("Audubon's Warbler") have yellow throat. The "Audubon's" that breeds in the interior is blacker above and below. All females and fall Males are duller than breeding males but show same basic patern. Abundant in coniferous or mixed woodlands. Variable song, a slow warble, usually rising or falling at the end in "Audubon's," a musical trill in "Myrtle." "Myrtle" is fairly common in winter in the west; "Audubon's" is casual in the east. 2nd Edition