1. Emlen, S.T.: Problems in identifying bird species by radar signature analyses: Intra-specific variability. In: The Biological Aspects of the Bird/Aircraft Collision Problem. Gauthreaux, S. (ed.), Air Force Office of Scientific Research, Clemson, South Carolina, 1974, pp. 509–524
2. Emlen, S.T.: Migration: Orientation and Navigation. In: Avian Biology. Farner, D.S., King, J.R. (eds.) V, 1975, pp. 129-219
3. Griffin, D.R.: Nocturnal bird migration in opaque clouds. NASA Spec. Publication SP-262, 169-188 (1972)
4. Griffin, D.R.: Oriented bird migration in or between opaque cloud layers. Proc. Am. Phil. Soc. 117, 117–141 (1973)
5. Keeton, W.T.: The orientational and navigational basis of homing in birds. In: Recent Advances in the Study of Behavior 5, 47–132. New York: Academic Press, 1974