Blue-gray Gnatcatcher. The undertail is completely white and white extends through the undertail coverts.
What's faster than a speeding warbler? That would be this little bird, a Blue-gray Gnatcatcher, a very hyperactive bird found in winter in far southern areas of the country, including FL. Here in Ding Darling, NWR on Sanibel, you can see them flitting through the mangroves, hunting for small insects in the leaves.
"The adult male in summer has a thin black eyebrow, mostly in front of eye and extending to forehead"(SFG) but not in winter. In summer their breeding range extends throughout much of the country.
I love the way the top photograph shows the full tail fanned, something they do often. The outer tail feathers are mostly white. In the bottom photo you can see how, when they close their tail, it's like a fan closing. The two outer tail feather come together and when the tail is fully closed it would appear all white from underneath. On this bird, there is white extending all the way from the tail feathers continuing down through the fluffy white undertail coverts. This may help ID it as to subspecies.
There are 3 subspecies, "caerula (KS-cent. TX and east); m. back bluish gray, undertail white through undertail coverts"... obscura (w. TX-WY and west); m. back darker undertail mostly white but black just before the undertail coverts... deppei (s. TX) like caerula but smaller" (from The Stokes Field Guide to the Birds of North America.)
In today's world of digital photography, where many a birder carries a digital camera, it becomes possible to see intimate details of a bird's plumage and tell things about it never before possible. Look closely at your digital photos, blow them up on your screen. Our new Stokes field guide will help you discover new things about the birds you are seeing.
As to getting a photo of this hyper species, moving through mangroves, well, that's a topic for another blog post.