Food intake and fuel deposition in a migratory bird is affected by multiple as well as single-step changes in the magnetic field

Ian Henshaw,Thord Fransson,Adrian Vallin,Johan Lind,Sven Jakobsson,Cecilia Kullberg

doi:10.1242/jeb.014183

Ian Henshaw, Thord Fransson + Show 4 more

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https://doi.org/10.1242/jeb.014183

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Abstract

Recent studies have shown that migratory thrush nightingales (Luscinia luscinia) experimentally treated with multiple changes of the magnetic field simulating a journey to their target stopover area in northern Egypt, increased fuel deposition as expected in preparation to cross the Sahara desert. To investigate the significance of food intake on the body mass changes observed, in the work described here we analysed food intake of the nightingales under study in those earlier experiments. Furthermore, to study whether a single change in the magnetic field directly to northern Egypt is sufficient to provide information for fuelling decisions, we performed a new experiment, exposing thrush nightingales trapped in Sweden, directly to a magnetic field of northern Egypt. Our results show that an experimentally induced magnetic field of northern Egypt, close to the barrier crossing, triggers the same response in fuel deposition as experiments with multiple changes of the magnetic field simulating a migratory journey from Sweden to Egypt, suggesting that migratory birds do not require successive changes in field parameters to incorporate magnetic information into their migratory program. Furthermore, irrespective of experimental set up (single or multiple changes of the magnetic field parameters) increase in food intake seems to be the major reason for the observed increase in fuelling rate compared with control birds, suggesting that geomagnetic information might trigger hormonal changes in migratory birds enabling appropriate fuelling behaviour during migration.

Highlights

Migratory flights in many songbirds are performed alone nocturnally using a combination of innate (Beck and Wiltschko, 1988) and external information about migratory direction (Alerstam, 1990; Berthold, 1996)
To study whether a single shift in magnetic field value is sufficient to provide information for fuelling decisions we performed a new experiment, exposing thrush nightingales trapped in Sweden, directly to a magnetic field of northern Egypt
All birds increased in body mass during the experiment and overall experimental birds showed a higher body mass increase than controls (ANOVA with body mass increase from day·1 as repeated measurement with 10 levels: F1,12=4.98, P=0.04)

Summary

Introduction

Migratory flights in many songbirds are performed alone nocturnally using a combination of innate (Beck and Wiltschko, 1988) and external information about migratory direction (Alerstam, 1990; Berthold, 1996). Migratory flights over the Sahara desert involve distances of at least 1500·km, and birds can double their mass (100% increase relative to lean body mass) by storing fat prior to crossing the desert (Fry et al, 1970). Recent evidence strongly suggests that temporal and spatial precision of bird migration incorporates additional external cues along with the inherited migratory program (Fransson et al, 2001; Thorup and Rabøl, 2001; Jenni and Schaub, 2003; Fransson et al, 2005; Alerstam, 2006; Kullberg et al, 2007). In order to find confined species-specific areas it seems likely that birds must use some additional external cues in combination with clock-and-compass orientation (Fransson et al, 2005)

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