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

Abstract

SUMMARYRecent 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.

Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird

“…during reproduction (Romero et al, 1997) and migration (Ramenofsky et al, 1995;Holberton et al, 1996), inhibit the adrenocortical response to environmental stressors, thus allowing the animal to carry on with breeding and feeding activities, respectively, during stressful conditions. Accordingly, and just as in earlier studies of thrush nightingales experiencing a simulation of the geomagnetic field of northern Egypt, the elevated fuel deposition rate seems to be due to an increase in food intake (Henshaw et al, 2008). The reduced adrenocortical response could thus be an adaptive response enabling birds to spend more time feeding in spite of stressful events (e.g.…”

“…First year thrush nightingales (Luscinia luscinia) exposed to a magnetic treatment simulating a migratory flight from southeast Sweden to northern Egypt increase food intake and attain a higher fuel load compared with control birds experiencing the ambient magnetic field in southeast Sweden (Fransson et al, 2001;Kullberg et al, 2003;Henshaw et al, 2008). These results are in line with the suggestion that migratory birds use external cues to get more precise information than the endogenous time programme alone would allow in deducing their latitudinal position (Gwinner, 1996;Thorup and Rabøl, 2001;Fransson et al, 2005), and suggest that geomagnetic information might act as an external synchronizer affecting hormonal secretions during migration, leading to adaptive fuelling behaviour along the migratory route (Henshaw et al, 2008). In order to investigate the effect of geomagnetic information on corticosterone secretion in migratory birds we here repeated our earlier studies and also measured baseline corticosterone levels and the adrenocortical response in juvenile thrush nightingales trapped in southeast Sweden during autumn migration and exposed to either the ambient magnetic field of southeast Sweden or the magnetic field of northern Egypt.…”

Crossing the desert barrier: Migration ecology of the Lesser Whitethroat (Sylvia curruca) at Eilat, Israel

“…During the initial stages of migration, fat stores may reach between 40 and 70% of lean body mass (Alerstam and Lindstrom, 1990). Henshaw et al (2008) demonstrated that food intake is the major reason for the increase in fueling rate, and that geomagnetic information triggers hormonal changes in migratory birds enabling appropriate fueling behavior during migration.…”

“…Several studies indicate that energetic condition influences probability and likelihood that a migrant will stay at a stopover site (Biebach et al, 1986;Moore and Kerlinger, 1987;Henshaw et al, 2008). An analysis of stopover length and energy accumulation patterns can provide insight into the migration strategies of a given species and their ability to reach their breeding grounds (Tøttrup et al, 2008).…”

Springtime migratory restlessness and departure orientation of Great Knots (Calidris tenuirostris) in the south compared to the north Yellow Sea

“…Regarding day length cues, captive birds can be induced into the migratory condition by manipulating day length (Landys et al 2004a). Regarding geomagnetic cues, simulated displacement along the migratory route by artificial changes in the geomagnetic field affects fuel preparation (Fransson et al 2001;Kullberg et al 2007;Henshaw et al 2008) and orientation behaviour (Henshaw et al 2010) of captive birds and thus may affect their motivation to resume migration. Probably by changing the plasma corticosterone level of the birds, these external cues could trigger the endogenous system to regulate the migratory restlessness (Landys et al 2004a;Lõhmus et al 2003).…”