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THE eXPerimental analysis of the orientational cues used by nocturnally migrating birds originated with Kramer's (1949) discovery that caged Blackcaps (Sylvia atricapilla ) and Red-backed Shrikes ( Lanius collurio ) would, under certain conditions, spontaneously orient their migratory activity or Zugunruhe toward the portion of the cage corresponding to the normal migration direction of the species.These preliminary studies were extended by Sauer (1957, 1961) and Sauer and Sauer (1960) who tested the directional responses of three sylviid species placed in cages under the natural night sky.The results indicated that some individuals consistently selected directions coincident with predicted migration routes; thus Sauer hypothesized that the birds were oftenting by means of celestial cues.Additional experiments, performed under the artificial skies of a small planetarium, provided strong support for this idea.Recently, several other species of nocturnal migrants have also been found to orient in "correct" migration directions when tested in circular cages out-of-doors (Merkel and Fromme,
Stephen T. Emlen (Sat,) studied this question.