Reference(s):

Lokki, J., Jännes, H. & Nikander, P. J., 1992: Otavan lintutieto.

–Kustannusosakeyhtiö Otava, Helsinki, Finland, 1992, pp. 1-440

      Translated from: Perrins, C. M. (ed.), 1990: The Illustrated encyclopaedia of birds.

–Headline Book Publishing Plc., London, England, 1990

Sibley, C. G. & Monroe, B, L., 1990: Distribution and taxonomy of birds of the world.

–Yale University Press, New Haven & London, 1990. xxiv-1111.