This Song Sparrow has no tail feathers. It may have lost its tail feathers through illness, irregular molt or a narrow escape from a predator. With regard to escape from a predator, a bird can go through a "fright molt", also known as a "stress molt" or "shock molt". During a fright molt, all the tail feathers are simultaneously ejected from their follicles, leaving the bird tail-less. By doing this the bird is able to escape a predator by leaving it with a mouthful of just feathers or confusing it by leaving some feathers behind.
In the case of this bird it may also be fledgling bird with no or very short tail feathers yet. This may be the case, because this the Song Sparrow still seems some of the down of a baby bird on it. If this is the case, the fledgeling's tail will grow and it will learn how to fly.
