This molt is a complete prebasic molt, and includes not just body feathers but also all the flight feathers. As you can see, this bird is pretty pathetic looking.
It was molting 4 of its 9 primaries (outer wing feathers), a few secondaries (the inner wing feathers), and all of its rectrices (tail feathers). Yes, it could still fly quite well!
At RRBO, we record the extent of molt on every bird. The number of body feathers being molted are scored on a 3-point scale, and we note the number of each molting flight feather. Over time, this gives us a great idea of the local annual timing of molt for the bird species we band.
Molt migration is common in waterfowl, and more common in songbirds in the western U.S. The other common molt migrant here is Swainson's Thrush (Catharus ustulatus); about 2% of RRBO's fall birds are molt migrants.
A little more information:
- An overview of bird molt -- Cornell's All About Birds
- Tennessee Warbler, featured bird -- Smithsonian Migratory Bird Center
- Tennessee Warblers are a classic example of a North American migrant that winters on shade coffee plantations -- Coffee & Conservation