Altered orientation and flight paths of pigeons reared on gravity anomalies: a GPS tracking study

Abstract

The mechanisms of pigeon homing are still not understood, in particular how they determine their position at unfamiliar locations. The "gravity vector" theory holds that pigeons memorize the gravity vector at their home loft and deduct home direction and distance from the angular difference between memorized and actual gravity vector. However, the gravity vector is tilted by different densities in the earth crust leading to gravity anomalies. We predicted that pigeons reared on different gravity anomalies would show different initial orientation and also show changes in their flight path when crossing a gravity anomaly. We reared one group of pigeons in a strong gravity anomaly with a north-to-south gravity gradient, and the other group of pigeons in a normal area but on a spot with a strong local anomaly with a west-to-east gravity gradient. After training over shorter distances, pigeons were released from a gravitationally and geomagnetically normal site 50 km north in the same direction for both home lofts. As expected by the theory, the two groups of pigeons showed divergent initial orientation. In addition, some of the GPS-tracked pigeons also showed changes in their flight paths when crossing gravity anomalies. We conclude that even small local gravity anomalies at the birth place of pigeons may have the potential to bias the map sense of pigeons, while reactivity to gravity gradients during flight was variable and appeared to depend on individual navigational strategies and frequency of position updates.

Figure 1. Magnetic and gravity anomalies around the pigeon lofts.

The location of the pigeon lofts, Zavallia and Savran, are indicated with a circle. (A) Magnetic anomaly map (nT = nanoTesla). (B) Gravimetric anomalies, the change of the gravity intensity (ΔG_B – Bouguer, mGal = milligal). Arrows show the direction of the gravity gradients. (C) Horizontal gravity gradients (E = Eötvös), highest values mark locations with steepest gradient of gravimetric values in border zones of gravimetric anomalies. Note the location of the Savran loft (S-pigeons) on a small yet steep gravity gradient in east-west direction characterized by elevated E-values. For a photographic map illustrating the topography of the area see https://www.dropbox.com/sh/2yrhdtcyzt5uu99/ZFJeNJb0lk.

Figure 2. Gravity and magnetic anomalies of the test region crossed by the pigeons.

(A) Gravimetric anomalies, the change of the gravity intensity (ΔG_B – Bouguer, mGal = milligal); (B) Horizontal gravity gradients (E = Eötvös), highest values mark locations with steepest gradient of gravimetric values in border zones of gravimetric anomalies. (C) Magnetic anomaly map (nT). The square symbols indicate the release sites: Pologi, the experimental release site; TZ, the last training flight to Zavallia loft; and TS, the last training flight to Savran loft. Black lines indicate beelines from the experimental release site to the Z- loft (46 km) and the S-loft (54 km).

Figure 3. Vanishing bearings of Z- and S-pigeons at 2 and 5 km.

(A) Vanishing bearings of Z- and S-pigeons at 2 km and (B) at 5 km from the release site. The black symbols refer to Z-pigeons (n = 12) whereas the white symbols refer to S-pigeons (n = 11). Circles, rectangles and triangles indicate pigeons released on August 26, 27 and 28, respectively. The bold arrows show the mean vanishing bearings of the Z-pigeons with a black arrow head and of the S-pigeons with a white arrow head. The dotted line shows the home loft direction, 152°. α is the mean vanishing bearing, r is the mean vanishing vector and hc is the homeward component. The difference between vanishing bearings of the Z- and the S-pigeons was calculated with the Watson-Williams-F-Test for significance (p-values in the circular diagrams). The significance levels for the Rayleigh test (r) are indicated with ¹ = not significant, * = p<0.05, ** = p<0.01.

Figure 4. Pigeons from the two lofts maintain different flight directions.

(A) Distances of the Z-pigeons from the beeline at 5 km steps. (B) Distances of the S-pigeons from the beeline at 5 km steps. Points easterly of the beeline R-H correspond to positive values of the x-axis, whereas points westerly of the beeline R-H correspond to negative values. The box ranges show the upper and lower quartile with the median, and whiskers extend to the most extreme data point no more than 1,5× the interquartile range. Points outside the range are outliers. The stars indicate significant differences between the Z- and the S-pigeons (t-test, p<0.05 for all).

Figure 5. Flight tracks from the same experimental release site.

The dotted line is the beeline from the release site, indicated with a star, to the home lofts. (A) The flight tracks of 12 Z-pigeons flying to their home loft Z (Zavallia). The distance from the release site, to the Z-loft is 46 km. (B) The flight tracks of 11 S-pigeons flying to their S-loft (Savran). The distance from the release site to the S-loft is 54 km. An orange dot indicates that the pigeon stopped flying and was pausing until the GPS ran out of battery power. A green dot indicates that the pigeon was still flying while the GPS ran out of battery power. The bars in both pictures represents 5 km.

Figure 6. Flight tracks of Z-pigeons crossing the Bandurove anomaly.

(A) Flight tracks of 11 Z-pigeons. Z depicts the home loft. The thick meandering blue line is the Bug river. The contour lines of the gravity anomaly (horizontal gradient) are in steps of 10 E. The brightness of the color denotes the anomaly intensity: light (E = 20), middle (E = 30), dark (E = 40). 1 E = 0.1 mGal/km. (B) Close-up of the same map but with blue contour lines in steps of 5 E. Zone 1: normal gravity area before the Bandurove anomaly; zone 2: gravity anomaly area with a steep change of the horizontal gravity gradient (isolines are close together); zone 3: the core anomaly area with continous values of 40–50 E. The bar in the lower left corner of both pictures represents 3 km.

Figure 7. Flight path of a pigeon crossing gravity and magnetic anomalies.

(A) Topographical map of Sekretarka region. (B) Map showing flight speed of pigeon 305. Note the sudden reduction in speed when approaching the anomaly; reduced flight speed is then maintained throughout the region. (C) Gravimetric anomalies. Densely spaced isolines indicate those regions with irregularities of the horizontal gravity gradient. (D) Strong magnetic peak on top of the gravitational anomaly. Asterisk denotes the position of a former Sovjet SS-18 rocket launch station.

Figure 8. Last training release of Z-pigeons northeast of the Zavallia loft.

(A) Flight tracks on gravity anomaly map. Most birds appear to follow the steepest gravity gradient of the anomaly, then turn left when they hit the Bug river. Scale 0–54 ΔmGal. (B) Flight tracks on magnetic anomaly map, showing partial coincidence of magnetic and gravity anomalies. Scale -500 to 5'500 nT. Violet arrow shows home direction. For a topographical map: https://www.dropbox.com/sh/2yrhdtcyzt5uu99/ZFJeNJb0lk.

Figure 9. Last training release of S-pigeons west of the Savran loft.

(A) Flight tracks on map of gravimetric anomaly, i.e. the change in gravity intensity. Yellow dots indicate birds resting more than 5 h. Three pigeons made long journeys to the north into the anomaly and back. Arrows show flight direction. (B) Tracks on a map showing irregularities of the horizontal gravity gradient (corresponding to the steepest gradients of gravimetric values). (C) Same tracks on a map with geomagnetic anomalies. The overall geomagnetic variation is relatively low (between -1000 and 1000 nT), with scattered peaks of higher intensity. For a topographical map: https://www.dropbox.com/sh/2yrhdtcyzt5uu99/ZFJeNJb0lk.