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. 2009 Sep 22;276(1671):3295-302.
doi: 10.1098/rspb.2009.0872. Epub 2009 Jun 25.

Do Release-Site Biases Reflect Response to the Earth's Magnetic Field During Position Determination by Homing Pigeons?

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Free PMC article

Do Release-Site Biases Reflect Response to the Earth's Magnetic Field During Position Determination by Homing Pigeons?

Cordula V Mora et al. Proc Biol Sci. .
Free PMC article

Abstract

How homing pigeons (Columba livia) return to their loft from distant, unfamiliar sites has long been a mystery. At many release sites, untreated birds consistently vanish from view in a direction different from the home direction, a phenomenon called the release-site bias. These deviations in flight direction have been implicated in the position determination (or map) step of navigation because they may reflect local distortions in information about location that the birds obtain from the geophysical environment at the release site. Here, we performed a post hoc analysis of the relationship between vanishing bearings and local variations in magnetic intensity using previously published datasets for pigeons homing to lofts in Germany. Vanishing bearings of both experienced and naïve birds were strongly associated with magnetic intensity variations at release sites, with 90 per cent of bearings lying within +/-29 degrees of the magnetic intensity slope or contour direction. Our results (i) demonstrate that pigeons respond in an orderly manner to the local structure of the magnetic field at release sites, (ii) provide a mechanism for the occurrence of release-site biases and (iii) suggest that pigeons may derive spatial information from the magnetic field at the release site that could be used to estimate their current position relative to their loft.

Figures

Figure 1.
Figure 1.
Circular distribution of second-order mean vanishing bearings of experienced (Frankfurt I, Frankfurt II, Tübingen) and inexperienced (Würzburg) homing pigeons relative to (a) the HD (ΔH) and (b) the direction of the steepest magnetic intensity slope at the point of vanishing (ΔM). Vanishing data were derived from four previously published figures for three lofts in Germany. (i) 49 release sites, Frankfurt loft I (fig. 1 in Grüter et al. 1982). (ii) 45 release sites, Frankfurt loft II (fig. 8 in Wiltschko 1992). (iii) 29 release sites, Tübingen loft (fig. 1 in Ganzhorn & Schmidt-Koenig 1988). (iv) 35 release sites, Würzburg loft (fig. 3.4 in Wallraff 2005). The direction of (a) home and (b) the steepest magnetic intensity slope is indicated by a dashed line. The overall mean vanishing vector for each dataset is indicated by a black arrow. Each distribution was tested for non-randomness with the Rayleigh test.
Figure 2.
Figure 2.
Sector analyses of ΔH and ΔM values. (a) Division of a circular diagram into six sector groups with each group encompassing 60°. For analysis of ΔH (mean vanishing direction in relation to HD), HD was standardized as 0° for each release site. For analysis of ΔM (mean vanishing direction in relation to the direction of the steepest magnetic field intensity slope at the point of vanishing), slope and contour directions were standardized as y- and x-axes, respectively. Percentage of (b) ΔH and (c) ΔM values that fell in each of the six sector groups defined in figure 2a for the four datasets. Significance levels for χ2 goodness-of-fit test testing the null hypothesis of equal distribution of ΔH and ΔM values across all six sectors: *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant.
Figure 3.
Figure 3.
(a) Circular distribution of mean vanishing bearings of individual releases for nine release sites (fig. 7 in Wiltschko 1992) transposed onto an aeromagnetic map showing magnetic field intensity contour lines at 5 nT intervals (pink and blue lines indicating deviations above and below the Earth's background field, respectively; all other lines not relevant to the current analysis). HD and distance from loft are listed below the panel for each site. Circles scaled to represent typical 2 km vanishing distance. Mean vanishing vectors for individual releases from each site: black arrows inside circle. Vanishing directions: symbols on circle's circumference (solid triangle, mean vanishing bearing showing a statistically significant bias; open triangle, mean vanishing bearing homeward orientated; open circle, mean vanishing bearing not significant according to Rayleigh test). HD, dashed line; median second-order vanishing vector (table 4 in Wiltschko 1992), bold red arrow. Sector analyses of (b) ΔH and (c) ΔM values combined for all individual releases from nine sites (fig. 7 in Wiltschko 1992). Only significantly oriented mean vanishing vectors (Rayleigh test) were included in this analysis. Percentage of (b) ΔH and (c) ΔM values that fell in each of the six sector groups defined in figure 2a. Significance levels for χ2 goodness-of-fit test testing the null hypothesis of equal distribution of ΔH and ΔM values across all six sectors: *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant.

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