Differential effects of magnetic pulses on the orientation of naturally migrating birds

Abstract

In migratory passerine birds, strong magnetic pulses are thought to be diagnostic of the remagnetization of iron minerals in a putative sensory system contained in the beak. Previous evidence suggests that while such a magnetic pulse affects the orientation of migratory birds in orientation cages, no effect was present when pulse-treated birds were tested in natural migration. Here we show that two migrating passerine birds treated with a strong magnetic pulse, designed to alter the magnetic sense, migrated in a direction that differed significantly from that of controls when tested in natural conditions. The orientation of treated birds was different depending on the alignment of the pulse with respect to the magnetic field. These results can aid in advancing understanding of how the putative iron-mineral-based receptors found in birds' beaks may be used to detect and signal the intensity and/or direction of the Earth's magnetic field.

Figure 1.

A schematic of the alignment of the pulse relative to the bird and the magnetic field or biasing field. (a) Perpendicular pulse. The bird is placed in the pulse coil with its head facing the direction of the pulse and no artificial biasing field. As the pulse coil is aligned west to east, the pulse is perpendicular to the only biasing field present, that of the Earth's magnetic field. This treatment shifts the orientation of migratory birds by 90° in orientation cages (e.g. Wiltschko et al. 1994). (b) Antiparallel pulse. The bird is placed in the pulse coil with its head facing the direction of the pulse. An artificially produced biasing field is activated in the opposite direction to the pulse. This treatment reverses the swimming direction of magnetotactic bacteria (Blakemore et al. 1980), but results in a bi-modal east–west orientation in migratory birds in an orientation cage (Wiltschko et al. 2002). (c) Parallel pulse. The same as in (b), except the artificial biasing field is aligned in the same direction as the Earth's magnetic field. This treatment does not change the swimming direction of magnetotactic bacteria, but results in a bi-modal east–west orientation in birds in an orientation cage (Wiltschko et al. 2002).

Figure 2.

Departure dates of birds from each experimental group.

Figure 3.

Circular diagram of departure bearings of control (black triangles, n = 10) and experimental (open triangles, n = 6) robins treated with a perpendicular magnetic pulse. The arrows represent the mean bearings and vector lengths of each group.

Figure 4.

Circular diagram of departure bearings of control (black triangles, n = 13) and experimental (open triangles, n = 11) reed warblers treated with a perpendicular magnetic pulse. The arrows represent the mean bearings and vector lengths of each group.

Figure 5.

Box plots of mean times between tagging and departure. Lower and upper box limits represent the 25th and 75th percentiles and error bars represent the 10th and 90th percentiles. Dashed line represents the mean and solid line the median value.

Figure 6.

Circular diagram of departure bearings of control (black triangles, n = 13), parallel pulse-treated (grey triangles, n = 11) and antiparallel pulse-treated (open triangles, n = 13) reed warblers. The arrows represent the mean bearings and vector lengths of each group.