Although thrushes are shy and secretive, they especially enjoy spending time at the bath as you can see by watching this juvenile Swainson's Thrush taking a bath.
The white spots are juvenile feathers that will be replaced by darker adult feathers. The covert wing feathers will be the last ones to be replaced. Covert feathers add thickness to the front of the wing allowing air to move faster over the top of the wing.
You can read more about the Swainson's Thrush in the section called Backyard Birds.