Feathers

"Sirens combine women and birds in various ways. In early Greek art Sirens were represented as birds with large women's heads, bird feathers and scaly feet. Later, they were represented as female figures with the legs of birds, with or without wings, playing a variety of musical instruments, especially harps. The tenth century Byzantine encyclopedia Suda^[13] says that from their chests up Sirens had the form of sparrows, below they were women, or, alternatively, that they were little birds with women's faces. Birds were chosen because of their beautiful voices. Later Sirens were sometimes depicted as beautiful women, whose bodies, not only their voices, are seductive."

Wikipedia