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Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass

Henrik Mouritsen^* and Ole Næsbye Larsen

Centre for Sound Communication, Institute of Biology, Odense University, Campusvej 55, DK 5230 Odense M, Denmark

View larger version (28K): [in a new window]   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.

View larger version (42K): [in a new window]   Fig. 2. Sketch of the experimental equipment and set-up in the planetarium.

View larger version (12K): [in a new window]   Fig. 3. Orientation of 17 pied flycatchers tested under a rotating planetarium sky, where the stars were turned 180° in the middle of the night. (A) The orientation of the birds before the turn (first 5.5 h of the night). (B) The orientation of the same birds after the celestial cues were turned 180° (last 5.5 h of the night). (C) Comparison on an individual basis of the orientation after the turn relative to the orientation before the turn of the celestial cues. Each individual bird’s orientation before the turn is defined as 180°. Thus, a direction of 360° indicates a 180° turn in orientation of the individual bird, equivalent to the turn of the celestial cues. The inner and outer dashed circles indicate the radius of the group mean vector needed for significance (P<0.05 and P<0.01, respectively) according to the Rayleigh Test of uniformity. mN, magnetic north; cN, celestial north.

View larger version (11K): [in a new window]   Fig. 4. Whole-night orientation of the 12 pied flycatchers that were tested under a stationary planetarium sky, and whose hourly orientations form the basis for Fig. 5. The birds oriented in a southerly direction, not significantly different from the expected autumn migratory direction. Each dot at the circle periphery indicates the mean orientation of one individual bird. The arrow indicates the group mean vector. The dashed circle indicates the radius of the group mean vector needed for significance (P<0.05) according to the Rayleigh Test of uniformity.

View larger version (22K): [in a new window]   Fig. 5. Orientation of young pied flycatchers tested under a stationary planetarium sky for 11 h. The black dashed (constant compass course routes) and the black dash-dotted (great circle routes) curves represent the predicted orientations, if the birds could deduce geographical (map) information from the stars and corrected for their apparent displacement. The solid blue line represents the predicted orientations, if the birds could deduce geographical (map) information from the stars and corrected back towards the capture site. The solid green line with a -15° slope indicates the predicted orientation of the birds if they use a time-compensated star compass. The horizontal red line represents the predicted orientation of the birds if they use a time-independent star compass. The group’s mean headings during each 1 h interval are shown on the circular diagrams at the top. Each diagram corresponds to the point directly below it. The numbers in these diagrams give the number of birds that were active and oriented during each 1 h period. Directly below, the headings are plotted as circles with error bars indicating 95 % confidence intervals for the groups’ mean orientation, for easy comparison with the predictions of the different hypotheses. A solid circle indicates that the group-mean-vector was significant. Open circles indicate a non-significant group-mean-vector. The heavy solid black line indicates the best linear regression through the data points and the heavy black dashed line indicates the 95 % confidence interval for the regression. The data suggest that young pied flycatchers use a time-independent star compass, and that they are not able to deduce their geographical position from the rotational phase of the starry sky.

View larger version (12K): [in a new window]   Fig. 6. Whole-night orientation of the seven young blackcaps, tested under a stationary planetarium sky, and whose hourly orientations form the basis for Fig. 7. The birds oriented in a southern direction not significantly different from the expected autumn migratory direction. For an explanation of the figure construction, see legend to Fig. 4.

View larger version (24K): [in a new window]   Fig. 7. Orientation of young blackcaps tested under a stationary planetarium sky for 11 h. The data suggest that young blackcaps use a time-independent star compass, and that they are not able to deduce their geographical position from the rotational phase of the starry sky. For an explanation of the figure construction, see legend to Fig. 5.