In songbirds, nocturnal activity is believed to be a characteristic feature of migration. However, unlike
experimental conditions where the onset of nocturnal restlessness is defined as a shift of activity leading up
to the dark period, this behaviour has, until now, not been observed in natural conditions. Here we studied
the nocturnal behaviour of radio-tagged juvenile Eurasian reed warblers (Acrocephalus scirpaceus) during
the pre-migratory period. The birds started nocturnal flights at the age of 38 days, whereas migration did
not commence until they were at least 50 days old. The birds left their natal site by nocturnal flights and
repeatedly returned to it. Such shuttle movements suggest the existence of a previously unknown period of
nocturnal activity. Motivation to perform such night flights gradually increases with age. We relate the
function of these nocturnal pre-migratory flights to the development of a stellar compass, necessary for
detecting the compass direction towards winter quarters and for the formation of a navigational target,
which will be used during return (spring) migration.
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