Ten grams and 13,000 km on the wing
In the study, geolocators were used 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. The 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. The recorded migratory routes for the tracked birds was compared with simulated routes obtained from different compass mechanisms.
Of the birds that were fitted with geolocators, three males were recaptured. The recorded data showed that they all were very similar in the routes they took, in their use of stopover sites and in the overall timing of migration - covering amazing 13 000 kilometres to reach their wintering sites.
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.