The blue-gray body, white eye-rings, and white outer tail feathers are diagnostic. The diminutive size (not much larger than a hummingbird) and the persistent calls (see below) are distinctive. The male is darker blue than the female and displays a black forehead. No bird goes about his business in quite the same energetic manner. The entry in A Natural History of American Birds (1939) reads:
“The blue-gray flycatcher is a fidgety little midget. Tiny, slender and frail in appearance … its littleness and defenselessness and its air of innocence and artlessness at once enlist our sympathy and interest. It is exceedingly active and graceful and may be seen dashing and skipping about (in) the topmost branches of some tall tree, from which it launches itself in pursuit of some passing insect or hovers to pick off some small creature.”
The gnatcatcher is common in the Smokies region in mixed and deciduous forests below 2,500 feet from late March into early September. But many people have them nesting in their yard without realizing they are there. The quickest way to know when they’re present is to be familiar with their vocalizations. The entry in “North American Birds Online” (a subscription web site) reads:
‘Two song types may occur. Most descriptions and recordings of song are of complex, continuous, rambling jumbles of a variety of phrases (discrete collections of similar notes) including sharp chips, high-pitched whistles, mewing notes comparable to calls, and trilled series of high sharp notes. These songs are of at least 10 seconds duration and are softly uttered and difficult to hear at more than 50 yards. Complex songs (and some isolated calls) appear to be mimetic. Species presumptively mimicked include Blue Jay, American Crow, Gray Catbird, Tufted Titmouse, Rufous-sided Towhee, Yellow-throated Vireo, Yellow Warbler, and Pine Siskin. Contact calls mewing and sharp … best rendered verbally as ‘zeee’ or ‘spee’ or ‘chay.’”
The mnemonic that works best for me is a softly whispered “chee-chee-chee.”
For various songs and calls go to: http://musicofnature.com/category/birds/blue-gray_gnatcatcher/.