Fuel deposition and speed of early autumn migration of juvenile Bluethroats Luscinia s. svecica leaving their natal area in Swedish Lapland

Abstract

The period between independence and the early phase of autumn migration is a difficult period for juvenile birds and a relatively poorly known part of the annual cycle. We present data on post-juvenile moult, fuel deposition and speed of early autumn migration of "red-spotted" Bluethroats Luscinia s. svecica, studied during 14 years (1984—1997) in Ammarnäs, Swedish Lapland. The juvenile Bluethroats leave their natal site from early August onwards, while still being in active post-juvenile moult. They do not put on any noticeable fuel stores, probably because foraging conditions are generally poor. This is reflected in a very slow initial speed of migration (11–21 km/d) between the natal area and stopover sites in eastern Sweden.