Birds of New Zealand, 2nd ed. ii. London, 1888, pp. 154-160.
The Chatham Island bird is P. onslowi of H. O. Forbes (Ibis, 1893, p. 537), who discusses various other species. The American forms need further study.
P. L. Sclater, P.Z.S. 1882, p. 458.
Freeman and Salvin, Falconry, its claims, etc., London, 1859, pp. 327-349.
W. A. Forbes, P.Z.S. 1882, p. 210.