Differential orientation in Scandinavian willow warblers explained by genotype

Photo: Dimitar Dimitrov

The willow warbler (Phylloscopus trochilus) is a tiny passerine capable of flying thousands of kilometers during long-distance migrations from the temperate regions of Europe and Asia to Sub-Saharan Africa and back. Just like most European songbirds, during their first autumn migration these small birds are flying alone in the night guided by their endogenous migration program. Willow warblers breeding in southern and northern Scandinavia belong to two different subspecies. Due to the efforts of a group of scientists from Lund University both subspecies can now be distinguished by genetic markers. Ringing recoveries and stable isotopes show that the willow warblers from southern and northern Scandinavia migrate to different wintering areas in Africa.

In a recent paper by Ilieva et al. 2012 published in Behavioural Processes we used orientation cages to show that the autumn migratory orientation of the Scandinavian willow warblers depends on their origin. Genetically assessed southern and northern birds headed south-west and south-southeast, respectively. Depending on whether the birds were tested far from the coast or at the southern tip of the island of Öland, they oriented in a different way. Willow warblers tested in front of an ecological barrier, the Baltic Sea, were less active compared to the individuals tested inland, especially when overcast conditions were simulated by placing Plexiglas sheets on top of the cages. More individuals also showed reverse directions at the coast, a phenomenon often observed in coastal areas. The current experiments show the potential of using orientation cages to study inherited migratory directions in willow warblers. Future studies of the orientation of hybrids between both subspecies will be crucial for a broader understanding of the mechanisms underlying the determination of migration direction.

Mihaela