Fig. 1. (A) Map illustrating the apparent displacement that the experimental birds will experience during the night, if they use the rotational phase of the starry sky to determine geographical position. The black arrows (Hypothesis 1a) show the predicted headings for each hour of the night, if the birds (pied flycatchers) compensate for the apparent displacement by orienting towards the intermediate goal in southern Spain along a constant compass course route (loxodrome). The blue arrows (Hypothesis 1c) show the predicted headings, if birds compensate by orienting back towards their true location (the location where the birds must expect to be that night). The red arrows (Hypothesis 3) show the constant orientation in an innate migratory direction expected if birds use the stars for a time-independent compass. (B) The directional choices expected in the computer-controlled funnels as a function of time for each of the five hypotheses (H) outlined in the introduction. H1a, black dashed curve; H1b, black dash-dotted curve; H1c, blue curve; H2, green line; H3, red line. Positive ordinate values indicate directions to the west of the normal migration direction, while negative values indicate directions to the east of it. For further explanation, see text.
