The nightingale—referred to by its name only in the poem’s title—is the hyperbolic “immortal Bird” and the center (Line 61) of the poem’s universe. As the adjective suggests, the nightingale is a symbol of immortality. Immortality is a complex theme in the poem, since it doesn’t imply literal deathlessness—a nightingale is after all as much a mortal creature as a human being. Instead, the immortality the nightingale represents is freedom from the constraints of human existence. The nightingale’s flight is compared to the movement of a dryad, a magical tree spirit, who can change from tree to a feminine form. Thus, the flight represents a blurring of boundaries and a freedom from shackles. The nightingale and its song symbolize a kind of timelessness that Keats associates with a pastoral existence. Unlike humans, whose “hungry generations” (Line 62) prey on each other, the nightingale is free from emotions like greed and the burden of thought. Although factually, a bird too would be susceptible to death and decay as the humans in whom “palsy shakes a few, sad, last gray hairs” (Line 25). Symbolically, the bird, in its lack of sentience, is unencumbered by the burden of mortality. That is to say, the bird is immortal simply because it does not know it’s going to die the way human beings possess this knowledge.