Magnetic cues and time of season affect fuel deposition in migratory thrush nightingales (Luscinia luscinia)

Abstract

Bird migration requires high energy expenditure, and long-distance migrants accumulate fat for use as fuel during stopovers throughout their journey. Recent studies have shown that long-distance migratory birds, besides accumulating fat for use as fuel, also show adaptive phenotypic flexibility in several organs during migration. The migratory routes of many songbirds include stretches of sea and desert where fuelling is not possible. Large fuel loads increase flight costs and predation risk, therefore extensive fuelling should occur only immediately prior to crossing inhospitable zones. However, despite their crucial importance for the survival of migratory birds, both strategic refuelling decisions and variation in phenotypic flexibility during migration are not well understood. First-year thrush nightingales (Luscinia luscinia) caught in the early phase of the onset of autumn migration in southeast Sweden and exposed to a magnetic treatment simulating a migratory flight to northern Egypt increased more in fuel load than control birds. By contrast, birds trapped during the late phase of the onset of autumn migration accumulated a high fuel load irrespective of magnetic treatment. Furthermore, early birds increased less in flight-muscle size than birds trapped later in autumn. We suggest that the relative importance of endogenous and environmental factors in individual birds is affected by the time of season and by geographical area. When approaching a barrier, environmental cues may act irrespective of the endogenous time programme.

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

“…The lack of a complete response in the experimental birds resembling natural fuelling patterns is probably due to constraints in the experimental set-up (for example the very fast change to the magnetic field of Egypt and the absence of all other natural cues birds will obtain during migration) (Kullberg et al, 2003). Corticosterone levels in birds are known to vary with social and environmental factors (Raof et al, 2006;Rubenstein, 2007;JenniEiermann et al, 2008), but observed seasonal variation in corticosterone levels has been suggested to be mediated through photoperiodically induced seasonal changes in physiology rather than variation in environmental factors like temperature, storms or habitat condition (Romero et al, 1997;Romero and Wingfield, 1998;Romero, 2002;Landys et al, 2004b;Romero and Rich, 2007;Holberton et al, 2008).…”

“…The magnetic field of northern Egypt was calculated according to IGRF 10th generation (International Association of Geomagnetism and Aeronomy, 2005). The chosen location in northern Egypt is based on autumn ring recoveries of thrush nightingales in the eastern Mediterranean area ringed in northern Europe, suggesting that they prepare for crossing the Sahara desert in a confined species-specific area (Fransson et al, 2005;Kullberg et al, 2003). In total eight birds were subject to the magnetic treatment and eight birds were used as controls.…”

“…Recent studies have shown that information from the geomagnetic field affects fuelling behaviour in migratory birds. 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).…”

Changes in brain gene expression during migration in the white-crowned sparrow

“…It is possible that, in addition to having evolved a number of behavioral, physiological and morphological adaptations to the demands of migration, (Dawson et al 1983;Andrews 1995;McWilliams and Karasov 2001;Kullberg et al 2003;Mettke-Hofmann and Gwinner 2003;Wikelski et al 2003;Mouritsen and Ritz 2005;Pravosudov et al 2006) migratory birds have also evolved adaptations to combat the negative consequences of sleep loss. To date, however, evidence of potential mechanisms for sleep loss compensation in migratory birds is absent.…”

Passerine migration strategies and body mass variation along geographic sectors across East Africa, the Middle East and the Arabian Peninsula

“…This is in agreement with a recent appraisal and study of spatio-temporal precision in bird migration. For instance, in an experimental study, Fransson et al (2001) and Kullberg et al (2003Kullberg et al ( , 2007 illustrated that birds may use geomagnetic cues as external indicators for successful migration, and they explain that these cues (simultaneously operating with bird's orientation behavior and endogenous rhythm) provide information about the geographical position when such information is required, for instance, for a fueling decision. Given this, depending on the species and the route taken, a certain relationship would be expected between body mass and latitude of a given geographic area.…”