Pigeons combine compass and landmark guidance in familiar route navigation

Abstract

How do birds orient over familiar terrain? In the best studied avian species, the homing pigeon (Columba livia), two apparently independent primary mechanisms are currently debated: either memorized visual landmarks provide homeward guidance directly, or birds rely on a compass to home from familiar locations. Using miniature Global Positioning System tracking technology and clock-shift procedures, we set sun-compass and landmark information in conflict, showing that experienced birds can accurately complete their memorized routes by using landmarks alone. Nevertheless, we also find that route following is often consistently offset in the expected compass direction, faithfully reproducing the shape of the track, but in parallel. Thus, we demonstrate conditions under which compass orientation and landmark guidance must be combined into a system of simultaneous or oscillating dual control.

Fig. 1.

Average anticlockwise deviation in the tracks of clock-shifted birds compared with the same birds' final non-clock-shifted flight, measured as virtual vanishing bearings (20) recorded at 500-m intervals throughout the length of the homing flight. Filled circles show mean deviations of high-familiarity birds; open circles show means of low-familiarity birds. Error bars are circular standard errors of the mean. Dashed lines indicate the expected deviation induced by the clock-shift treatment at each site, based on the magnitude of the shift, date and time of release, and geographical location. (A) Weston Wood. (B) Hinksey Heights. (C) High Cogges. (D) Bladon Heath. At the 2,000-m boundary (the distance at which vanishing bearings are customarily measured), the distributions of angular differences in the low- and high-familiarity groups were significantly different at three of the four sites (Watson's U² test: P < 0.005 at High Cogges; P < 0.05 at Bladon Heath and Weston Wood; P > 0.05 at Hinksey Heights).

Fig. 2.

Nearest-neighbor vector analysis. (A) The calculation of nearest-neighbor vectors is illustrated through an example bird. For each point on the analyzed track (here the “clock-shifted flight”) a vector was drawn to the nearest time-independent neighbor among the points recorded during previous flights by the same bird. Axes show U.K. Ordnance Survey northings and eastings. (B) The nearest-neighbor vectors are plotted for the example bird in A. Each vector is rotated such that the estimated instantaneous trajectory of flight is 0°; the scale for vector lengths is shown by concentric circles. Vectors between non-clock-shifted releases are shown in blue; vectors from the clock-shifted release to nonshifted tracks are shown in red. (C) The distributions of mean vectors for birds in the three groups are examined: non-clock-shifted flights of high-familiarity birds (blue), clock-shifted flights of high-familiarity birds (red), and clock-shifted flights of low-familiarity birds (green). Second-order mean and covariance ellipse are shown for each distribution. The three groups' distributions are significantly different (two-sample 2D Kolmogorov–Smirnov test, n = 133, n = 27, n = 31, respectively; D > 0.45, P < 0.01 for all pairwise comparisons). (D) The mean vectors of high-familiarity birds' clock-shifted flights are compared with the distribution of their previous, non-clock-shifted flights (blue ellipse). Red dots show mean vectors that are significantly unlikely to have come from the latter distribution (see also Fig. 3); gray dots show mean vectors that cannot be statistically distinguished from non-clock-shifted performance (calculated by using the Mahalanobis distance, with significance threshold at P = 0.05). In C and D, Insets magnify the central parts of the plots.

Fig. 3.

Flight tracks of high-familiarity birds whose navigational performance was found to be significantly affected (A) or unaffected (B) by clock-shift. Each set of tracks, denoted by Arabic numerals, corresponds to a single bird's final three training flights from a given release site before clock-shifting (blue tracks) and a single flight after clock-shift treatment (red track). Tracks from the four release sites are rotated such that the release point is always at the top, and the location of the home loft is at the bottom in each set of tracks. Tracks are arranged in order of decreasing Mahalanobis distance from the distribution of non-clock-shifted high-familiarity birds (blue ellipse in Fig. 2 C and D). (Scale bar, 1 km.)

Fig. 4.

Performance of all high-familiarity birds at the Weston Wood release site, immediately before (blue tracks) and after (red tracks) the clock-shift treatment. Open circle indicates location of the release site; filled circle shows location of home. (Scale bar, 1 km.)