It is fascinating how birds really get into taking a bath.
When birds bathe in water or saturate themselves with dust they are actively maintaining their plumage. In well-watered areas bathing is most common, in arid ones dusting is more often observed.
In a typical sequence a bird stands in the water, fluffs the feathers to expose the bare skin between their bases, and rapidly flicks the wings in and out of the water. The breast is submerged and rolled vigorously back and forth, and then, as the front end emerges, the head is thrown back, forming a cup with the partially elevated wings and tail, and dousing the feathers of the back. Those feathers are elevated so that the water reaches the skin, and then lowered, forcing the water between them. The sequence may be repeated, with the bird submerging farther in each cycle, until it is a mass of soaked disarranged feathers. Variations on this theme can be seen in different species, such as robins, thrushes, mockingbirds, jays, and titmice.