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.

A magnetic pulse does not affect free-flight navigation behaviour of a medium-distance songbird migrant in spring

“…Therefore, it is not obvious why such landmarks would be more important at our test site than at others. It remains unclear why free-flying birds in Holland (2010) and Holland and Helm (2013) responded to the magnetic pulse with a deflected departure direction, but not in our present study or in Karwinkel et al (2022).…”

“…We consequently expanded our study to further explore the role of distance to final destination on magnetic map navigation and repeated our pulsing study with the same methodology on a short to medium distance migrant, the European robin ( Erithacus rubecula , hereafter robin). During spring stopover on Helgoland, robins are much closer to their migratory destination [some 50 to 1250 km], in this case to their breeding sites (Dierschke et al 2011), compared to wheatears in our former study, whose destination was their African wintering grounds (Karwinkel et al 2022). Importantly, in an independently conducted study, free-flying robins have been reported to be affected by magnetic pulse pre-treatment on spring migration (Holland 2010).…”

“…The total weight of the radio-transmitters including harness was about 0.31 g, which did not exceed 2.0% of the bird’s body mass (median: 1.8 %) and is therefore well below the 3–5% recommended threshold (Kenward 2001). To track the behaviour of the birds in free flight, we used an automated radio-receiving system on Helgoland (Müller et al 2018; Karwinkel et al 2022). This system consists of four SensorGnome receivers (www.sensorgnome.org) at three sites on Helgoland, connected to 16 radially aligned antennas (6-element Yagi antennas, Vårgårda Radio AB, Sweden), ensuring a radial resolution of 22.5° (Fig.…”

“…Therefore, the results of the pulse studies are consistent with a magnetic-particle-based sensor in a geomagnetic map navigation context. Using a regional high-throughput tracking system with unprecedented spatiotemporal resolution, we recently performed a pulse study comprising of 140 individuals of a long-distance migrant songbird, the northern wheatear (Oenanthe oenanthe, hereafter wheatear), on the offshore island of Helgoland in the German Bight during their autumn stopover (Karwinkel et al 2022). To our surprise, pulsed birds and control birds turned out to be statistically indistinguishable in all migration-related behavioural decisions we had monitored, i.e., departure directions, departure probability, nocturnal departure timing, and consistency in flight direction over 50-100 km (Karwinkel et al 2022).…”

“…Using a regional high-throughput tracking system with unprecedented spatiotemporal resolution, we recently performed a pulse study comprising of 140 individuals of a long-distance migrant songbird, the northern wheatear ( Oenanthe oenanthe , hereafter wheatear), on the offshore island of Helgoland in the German Bight during their autumn stopover (Karwinkel et al 2022). To our surprise, pulsed birds and control birds turned out to be statistically indistinguishable in all migration-related behavioural decisions we had monitored, i.e., departure directions, departure probability, nocturnal departure timing, and consistency in flight direction over 50–100 km (Karwinkel et al 2022). Taken all previous results together, the most parsimonious explanation for the absence of a pulse effect is the dispensability of magnetic map cues for the wheatear, at least at this offshore location.…”