Féré, "Perversions sexuelles chez les animaux," Revue Philosophique, May, 1897.
Tillier, L'Instinct Sexuel, 1889, p. 270.
Moll, Libido Sexualis, Bd. I, p. 76. The same author mentions (ibid., p. 373) that parrots living in solitary confinement masturbate by rubbing the posterior part of the body against some object until ejaculation occurs. Edmund Selous ("Habits of the Peewit," Zoölogist, April, 1902) suggests that the peewit, when rolling on the ground, and exerting pressure on the anal region, is moved by a sexual impulse to satisfy desire; he adds that actual orgasm appears eventually to take place, a spasm of energy passing through the bird.