Abstract
BackgroundHigh-latitude bird migration has evolved after the last glaciation, in less than 10,000–15,000 years. Migrating songbirds rely on an endogenous migratory program, encoding timing, fueling, and routes, but it is still unknown which compass mechanism they use on migration. We used geolocators to track the migration of willow warblers (Phylloscopus trochilus yakutensis) from their eastern part of the range in Russia to wintering areas in sub-Saharan Africa. Our aim was to investigate if the autumn migration route can be explained by a simple compass mechanism, based on celestial or geomagnetic information, or whether migration is undertaken as a sequence of differential migratory paths possibly involving a map sense. We compared the recorded migratory routes for our tracked birds with simulated routes obtained from different compass mechanisms.ResultsThe three tracked males were very similar in the routes they took to their final wintering sites in southern Tanzania or northern Mozambique, in their use of stopover sites and in the overall timing of migration. None of the tested compass mechanisms could explain the birds’ routes to the first stopover area in southwest Asia or to the destination in Southeast Africa without modifications. Our compass mechanism simulations suggest that the simplest scenarios congruent with the observed routes are based on either an inclination or a sun compass, assuming two sequential steps.ConclusionsThe birds may follow a magnetoclinic route coinciding closely with the tracks by first moving west, i.e. closer to the goal, and thereafter follow a constant apparent angle of inclination to the stopover site. An alternative would be to use the sun compass, but with time-adjustments along the initial part of the migration to the first stopover, and thereafter depart along a new course to the winter destination. A combination of the two mechanisms cannot be ruled out, but needs to be confirmed in future studies.
Highlights
High-latitude bird migration has evolved after the last glaciation, in less than 10,000–15,000 years
All three males departed from the breeding sites 17 to 21 August 2016 and arrived at an intermediate stopover site located in southwestern Asia about 5 weeks later; the exact locations are uncertain as this period coincided with the autumnal equinox (Fig. 1a)
We investigated whether any alternative compass mechanism based on celestial information [11] or information from the geomagnetic field [45] could explain the observed autumn migratory routes in willow warblers breeding in the eastern part of the range
Summary
High-latitude bird migration has evolved after the last glaciation, in less than 10,000–15,000 years. Migrating songbirds rely on an endogenous migratory program, encoding timing, fueling, and routes, but it is still unknown which compass mechanism they use on migration. Current migratory routes to the most northerly breeding latitudes, have evolved over short time periods dating back no longer than the Pleistocene, following the last glaciation 10,000 to 15,000 years BP Performance of long migrations, and to timely arrive to explore distant resources available during limited time, are challenging for migratory birds involving sophisticated diurnal and circannual time-keeping [7]. Still many birds are capable of extremely long migrations, of which the willow warbler performs one of the longest among songbirds, weighing less than 10 g and migrating > 10,000 km one-way in one season [12]
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