No apparent effect of a magnetic pulse on free-flight behaviour in northern wheatears (Oenanthe oenanthe) at a stopover site

Abstract

Naïve migrants reach their wintering grounds following a clock-and-compass strategy. During these inaugural migrations, birds internalise, among others, cues from the Earth's magnetic field to create a geomagnetic map, with which they navigate to destinations familiar to them on subsequent migrations. Geomagnetic map cues are thought to be sensed by a magnetic-particle-based receptor, which can be specifically affected by a magnetic pulse. Indeed, the orientation of experienced but not naïve birds was compromised after magnetic pulsing, indicating geomagnetic map use. Little is known about the importance of this putative magnetoreceptor for navigation and decision-making in free-flying migrants. Therefore, we studied in unprecedented detail how a magnetic pulse would affect departure probability, nocturnal departure timing, departure direction and consistency in flight direction over 50-100 km in experienced and naïve long-distant migrant songbirds using a large-scale radio-tracking system. Contrary to our expectations and despite a high sample size (n_total = 137) for a free-flight study, we found no significant after-effect of the magnetic pulse on the migratory traits, suggesting the geomagnetic map is not essential for the intermediate autumn migration phase. These findings warrant re-thinking about perception and use of geomagnetic maps for migratory decisions within a sensory and ecological context.

Keywords: bird migration; geomagnetic map; magnetic pulse; magnetic-particle-based sensor; magnetoreception; navigation.

Figure 1.

(a) Estimated autumn migration routes of northern wheatears (Oenanthe oenanthe) passing the island of Helgoland (white dot). (b–d) Free-flight behaviour was recorded using radio telemetry: (b) Locations of automated radio-receiving stations on Helgoland (H) and along the coastline of the German Bight (yellow dots). The closest station to Helgoland is Wangerooge (W) in the south, about 44 km away. The radio-receiving stations on Sylt (S, about 72 km from Helgoland) and Borkum (B, about 104 km from Helgoland) encompass the remaining stations. Inset shows antennas of an example radio-receiving station. (c) On Helgoland, three automated radio-receiving locations with 16 antennas, evenly spaced about 22.5° apart in direction, are installed to precisely determine departure timing and direction from Helgoland. (d) An example of a nocturnal departure event as recorded by our system displaying signal strength over time (coordinated universal time: UTC). Colours correspond to antenna directions on Helgoland in (c). The plot starts with three parallel lines, indicating the bird being stationary. The following peak with increasing numbers of different antennas (colours) indicates the take-off. The decreasing number of antennas and signal intensity indicates the bird flying off the island in a specific direction until the signal is lost (see the electronic supplementary material for details). The grey dots after approximately 2 h indicate the passage at the coastline radio-receiving station in Fedderwardersiel (F in (b)), with the first detection defined as coastline arrival. Photos by T.K.

Figure 2.

Application of the control (a) and experimental treatment (b) to the birds. Birds were hand-held and the head was placed into an indentation of a foam block (light blue box, extruded polystyrene foam, XPS) fixed to a wooden table. The beak of the bird was immobilised by positioning it into a small piece of plastic tube (dark blue) in the foam. (a) The control group only experienced a short ‘click' sound, but all handling was similar to the experimental group. (b) The experimental group experienced a magnetic pulse from a small coil (grey cylinder; 50 mm diameter with 15 × 15 windings of 1 mm copper wire) with the coil's magnetic north pointing perpendicular to the beak. The distance between the beak and the coil was adjusted to yield a peak magnetic field of ca 0.1 T (100 mT). Heat map shows peak magnetic field intensity with isolines in white and selected magnetic field lines in black. All experiments were performed with the birds directed southwards (south anterior). (c + d) Time course of the magnetic field intensity of the magnetic pulse. The measurement was taken with a self-made pick-up induction coil (5 cm diameter, six turns) placed on the pulse coil and connected to an oscilloscope (Tektronix MDO3032, Beaverton USA). The induced voltage measured (c), which is proportional to the rate of change of the magnetic field with respect to time, was integrated to yield the time course of the magnetic field in Tesla (d). After triggering the pulse at t = 0, the magnetic field reaches its maximum at 1.5 ms to then decay within 8 ms.

Figure 3.

Free-flying migratory behaviour of juvenile (left column) and adult (right column) northern wheatears (Oenanthe oenanthe) after treatment with a magnetic pulse (red triangle scheme) compared to the control group (blue circle scheme), as observed by radio telemetry. Numbers indicate sample sizes. Sample sizes for every section decreased stepwise as not every trait could be assigned for every bird (see methods for details). (a) Departure probability as proportion of birds departing on the first night after the treatment from Helgoland (white numbers, lower bar) or staying for the first night (black numbers, upper bar). (b) Departure timing as minutes after sunset. Asterisk: one juvenile control and one adult experimental bird departed before sunset (−167 and −235 min after sunset, respectively). (c) Initial departure direction from Helgoland. (d) The consistency of flight direction after departure from Helgoland until passage at the coastline (50–100 km), given as the directional deviation between departure direction from Helgoland and passage location site on the coastline (see methods for details). Dashed lines in circular plots indicate 95% confidence intervals. Data points in the circular plots are shifted slightly off-centre by less than 5° between the groups to better distinguish the data of the corresponding groups. Photos by T.K.