Thrush Nightingale (Luscinia luscinia) Science Article 2
Bird migration requires high energy expenditure, and long-distance migrants accumulate fat for use asfuel during stopovers throughout their journey. Recent studies have shown that long-distance migratorybirds, besides accumulating fat for use as fuel, also show adaptive phenotypic flexibility in several organsduring migration. The migratory routes of many songbirds include stretches of sea and desert wherefuelling is not possible. Large fuel loads increase flight costs and predation risk, therefore extensive fuellingshould occur only immediately prior to crossing inhospitable zones. However, despite their crucial importancefor the survival of migratory birds, both strategic refuelling decisions and variation in phenotypicflexibility 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 magnetictreatment 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 highfuel load irrespective of magnetic treatment. Furthermore, early birds increased less in flight-muscle sizethan birds trapped later in autumn. We suggest that the relative importance of endogenous and environmentalfactors in individual birds is affected by the time of season and by geographical area. Whenapproaching a barrier, environmental cues may act irrespective of the endogenous time programme.
Cecilia Kullberg, Johan Lind, Thord Fransson, Sven Jakobsson and Adrian Vallin, Proc. R. Soc. Lond. B (2003) 270, 373-378