2902 INTRODUCTION Most songbirds perform their migration alone during nocturnal flights often over long distances. The main fuel for migratory flights is fat, deposited at stopover sites en route (Klaassen, 1996). As large fat stores entail increased flight costs, maintenance and increased predation risk (Alerstam and Lindström, 1990; Kullberg et al., 1996), most bird species accumulate rather small fat deposits (<25% of lean body mass) and refuel at several successive stopover sites (Alerstam and Lindström, 1990; Schaub and Jenni, 2000a; Schaub and Jenni, 2000b). When facing large ecological barriers like the Sahara desert that cover at least 1500 km with hardly any possibilities to refuel, extensive fuel loads (100% of lean body mass) are needed for a successful passage (Alerstam and Lindström, 1990; Fransson et al., 2008). In preparation for migratory flight birds develop a state of hyperphagia or over-eating (King, 1961; Bairlein, 1985) and, together with an increased efficiency of food utilization (Bairlein, 1985; Klaassen and Biebach, 1994), this provides an important mechanism for migratory fuelling. The annual timing of migration and normal seasonal progression of migratory disposition such as migratory restlessness and body mass increase has also been shown to appear in seasonal constant environments in the laboratory (reviewed by Gwinner, 2003; Gwinner and Helm, 2003). These apparent circannual rhythms have led to the assumption that juvenile songbirds rely on an inherited endogenous programme, fine tuned by photoperiod and possibly other synchronizers, defining the migratory journey in terms of direction, distance and fuelling behaviour along the route (Gwinner, 1990; Berthold, 1996). The behavioural and physiological changes during migration not only occur at the onset of the initial departure but also will be repeated throughout the journey during substages (Jacobs and Wingfield, 2000) of moderate to extreme fuelling bouts with feeding, mass gain and preparation for departure, and flight bouts of different lengths. Migratory birds will thus alternate between different physiological stages, varying from extremely fast energy deposition to prolonged bouts of energy expenditure (Jenni-Eiermann and Jenni, 1991; Jenni and Schaub, 2003). Even though the hormonal regulation of migration is not fully understood, probably being extremely complex and relying on several hormones and their relationships and interactions, the primary glucocorticosteroid hormone corticosterone seems to play a major role in controlling behavioural and physiological traits in migratory birds (Wingfield, 2003; Ramenofsky and Wingfield, 2007). Elevated corticosterone plasma levels have been shown for many species during the migratory period (Ramenofsky et al., 1995; Holberton et al., 1996; Romero et al., 1997; Piersma and Ramenofsky, 1998; Holberton, 1999; Piersma et al., 2000), and have been found to correlate with behaviours such as hyperphagia (Landys et al., 2004a; Lõhmus et al., 2006), migratory activity (Landys et al., 2004b; Holberton et al., 2007) and orientation behaviour (Lohmus et al., 2003). During the adrenocortical response to an acute stress, plasma levels of corticosterone can increase within minutes and enable behavioural and physiological adjustments redirecting the individual to life-saving activities during unpredicted stressful events (Wingfield et al., 1998). Some studies suggest elevated baseline corticosterone levels and a reduced adrenocortical response in migratory birds compared with pre-migratory birds (Holberton et al., 1996; Holberton, 1999; Long and Holberton, 2004). The migration modulation hypothesis (Holberton et al., 1996) suggests that during migration birds should express elevated baseline corticosterone levels to facilitate migratory fattening while the adrenocortical response should be reduced to prevent the already high level of plasma corticosterone becoming too high during a The Journal of Experimental Biology 212, 2902-2907 Published by The Company of Biologists 2009 doi:10.1242/jeb.033332 Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird Ian Henshaw 1 , Thord Fransson 2 , Sven Jakobsson 1 , Susanne Jenni-Eiermann 3 and Cecilia Kullberg 1, * 1 Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden, 2 Swedish Museum of Natural History, Bird Ringing Centre, Box 50 007, SE-104 05 Stockholm, Sweden and 3 Swiss Ornithological Institute, CH-6204 Sempach, Switzerland *Author for correspondence (cecilia.kullberg@zoologi.su.se) Accepted 18 June 2009 SUMMARY Long-distance migrants regularly pass ecological barriers, like the Sahara desert, where extensive fuel loads are necessary for a successful crossing. A central question is how inexperienced migrants know when to put on extensive fuel loads. Beside the endogenous rhythm, external cues have been suggested to be important. Geomagnetic information has been shown to trigger changes in foraging behaviour and fuel deposition rate in migratory birds. The underlying mechanism for these adjustments, however, is not well understood. As the glucocorticoid hormone corticosterone is known to correlate with behaviour and physiology related to energy regulation in birds, we here investigated the effect of geomagnetic cues on circulating corticosterone levels in a long-distance migrant. Just as in earlier studies, juvenile thrush nightingales (Luscinia luscinia) caught during autumn migration and exposed to the simulated geomagnetic field of northern Egypt increased food intake and attained higher fuel loads than control birds experiencing the ambient magnetic field of southeast Sweden. Our results further show that experimental birds faced a reduced adrenocortical response compared with control birds, thus for the first time implying that geomagnetic cues trigger changes in hormonal secretion enabling appropriate behaviour along the migratory route. Key words: bird migration, fuelling decisions, hormones, corticosterone, geomagnetic cues. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY