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Edited by John G. Hildebrand, University of Arizona, Tucson, AZ, and approved June 14, 2016 (received for review February 25, 2016)
Article Figures & SI
Figures
- Fig. 1.
Birds were video-recorded in a 5.5-m-long flight tunnel to study their course control strategies (Movie S1). An automated tracking system determined 3D flight trajectories from eight camera views (A). Sample trajectories are shown for a bird flying to the feeder and then back to the perch when both walls presented vertical gratings (B). Trajectories were analyzed between meters 1 and 5 within the tunnel to exclude the take-off, feeder approach, and landing phases. For illustration purposes, the bird, perch, and stimulus features are not to scale.
- Fig. 2.
Unlike insects, birds did not steer to balance nasal-to-temporal pattern velocity on their left and right sides. Motion of vertical stripes (A) and dot fields (B) did not affect flight trajectories as predicted if birds control flight using nasal-to-temporal pattern velocities (Movie S2). However, when horizontal stripes moved upward, simulating descent, the birds steered upward (C). Data points in the middle column show grand means for individual birds connected by solid lines (lateral position in A and B; altitudinal position in C). Pink and blue are used to represent the motion treatments and white represents the controls. To allow further comparison of the treatments, the shaded curves show the estimated probability densities for all of the flight averages. The area under each curve sums to a probability of one. Note that the tunnel midline has a lateral position of 0 and the walls are at lateral positions of ±29.5 cm (the axis range of ±30 cm in A and B is used for illustration only). Effect sizes, calculated as the average difference between each treatment and the control, are plotted in the rightmost column, with a separate effect size shown for each bird (small gray dots) and the overall effect size relative to the control and its 95% CI below. The analyses in A and B define left and right using the bird’s frame of reference. The gratings in A and C had a spatial period size of 18.4 cm. Statistical significance: ***P < 0.001; **P < 0.01; *P < 0.05; ns P > 0.05.
- Fig. 3.
Course control depends on the grating period size. Birds were predicted to steer away from vertical stripe gratings (and toward horizontal stripes) (A), but this was only observed when the stripe gratings had period sizes of 2.3 cm and 1.15 cm. Vertical stripe gratings with a period size <1 cm nearly always fused as a result of motion blur, although fusion cannot explain the diminished effect of the gratings with a large period (B). Pink is used to represent treatments with the horizontal grating on the left, whereas blue treatments have the horizontal grating on the right. The middle column shows grand means for the individual birds and probability densities for the flight averages, as in Fig. 2. Effect sizes are calculated as the average difference between the pink and blue treatments. Note that this analysis uses the tunnel frame of reference, because birds were expected to steer away from vertical gratings regardless of heading. Stimulus features are not drawn to scale. See SI Appendix for details of fusion calculations. Statistical significance: ***P < 0.001; **P < 0.01; ns P > 0.05.
- Fig. 4.
Lateral course control is based on the size of horizontal features, even in the absence of nasal-to-temporal pattern velocity. Hummingbirds had a strong tendency to steer away from vertical stripe gratings and toward horizontal gratings, but only when the horizontal stripes were relatively small (2.3-cm period size; top two rows of A). This steering response was diminished by the presence of large horizontal stripes (18.4-cm period size; bottom two rows of A). Additional tests confirmed that birds steered away from horizontal gratings with a larger period size even in the absence of vertical stripes that generate nasal-to-temporal pattern velocity (B) (Movie S2). The middle column shows grand means for individual birds and probability densities, as in Fig. 2. Effect sizes are calculated as the average difference between the pink and blue treatments. Stimulus features are not drawn to scale. Statistical significance: ***P < 0.001; *P < 0.05; ns P > 0.05.
- Fig. 5.
We propose that expansion may govern lateral course control in birds. Vertical expansion, ρv, is defined as the change in the angle, θ, subtended by a feature in the vertical axis (A). The rate of expansion increases much more rapidly for features with a greater vertical extent (i.e., height) (B). Expansion may be perceived as the birds move laterally; this is shown by the green and magenta lines in C and D, which indicate ρv at the point of expansion (±90° azimuth, 0° elevation) for a bird moving to the left (green) or right (magenta) at 0.1 m/s (which is typical of the maximum lateral flight speeds observed) (SI Appendix, Table S10). The dashed lines in C and D are the grand mean (average) lateral positions observed for birds that were halfway to the feeder. The shaded regions are the grand mean (average) extremes. Note that all flights started and ended on the midline of the flight tunnel, where lateral position = 0. In our experiments, the birds steered toward a position that would have reduced ρv relative to their starting position on the midline (C). When the features on both sides of the tunnel had the same vertical extent (height), the birds remained on average centered on the midline (D). See text in SI Appendix and SI Appendix, Table S10 for details.
Data supplements
Supporting Information
- Download Supporting Information (PDF)
- Download Appendix (PDF)
- Download Movie_S01 (MOV) - Examples of flight trajectories used to calculate the average lateral position of a bird as it crossed the tunnel. Although the tunnel is 5.5-m long, axial positions >5 m and <1 m are excluded from analysis. The animation does not indicate the actual flight speeds.
- Download Movie_S02 (MOV) - Flight trajectories from experiments testing moving and stationary gratings. Note that although this clip illustrates flights toward the feeder, the analysis also includes flights in the reverse direction on the way back to the perch. The animation does not indicate the actual flight speeds.
Visual guidance of forward flight
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