Inherited differences of migratory phenotypes in two Acrocephalus warblers in relation to geomagnetic field parameters

Birds have evolved morphological, physiological and behavioural adaptations, encoded in their migratory programs, to enable successful migration. Sometimes, even closely related species, such as the Eurasian reed warbler and the sedge warbler, may adopt different migration strategies to reach their wintering grounds in sub-Saharan Africa. To study them in detail, we aimed to compare the temporal variations of activity patterns and fuelling of lab-held warblers caught in southern Sweden during autumn migration and compared the results with respect to known differences in their migration strategies.In order to investigate if the activity and fuelling were controlled by magnetic information, two groups of birds per species were experimentally subjected to magnetic fields anticipated during their migration via the Mediterranean and at the winter destinations, and control experiments were performed in the local magnetic field. Both species expressed a peak of nocturnal activity just after sunset and increased activity before sunrise. Reed warblers were strictly active during the night, whereas sedge warblers also showed increased morning activity possibly associated with intense foraging during stopover and longer migratory flights extending into the day when passing ecological barriers. There was no difference in overall activity between the two species but a change of active intervals during the experimental period with reed warblers increasing and sedge warblers decreasing activity over time, probably reflecting similar anticipated migration distances but differences in migratory strategies. The simulation of magnetic parameters influenced the activity and fuelling in reed warblers, but not in sedge warblers. These results support inherited differences in migratory strategies observed in the wild and suggest species-specific adaptations to magnetic field information regulating migration.Significance statementMigratory birds often cover thousands of kilometres between their breeding and wintering sites. To successfully reach the destination, a bird may adopt a unique migration strategy including accumulation of fuel reserves and changes of diel activity pattern. Field observations have revealed how populations of birds perform their migrations; however, detailed monitoring of activity and fuelling over time for individual birds in a controlled setting is only possible in the laboratory. By using continuous video recording and automatic computer vision behavioural analysis, we quantified migratory activity patterns of two related species of warblers. We show that Eurasian reed warblers were strictly active during night, whereas sedge warblers also showed increased morning activity reflecting their migratory strategies observed in the wild. By simulating the magnetic field of sites along the migration route and wintering grounds, we found species-specific adaptations to magnetic field information.

Automated summary

Birds have developed adaptations in their migratory programs to ensure successful migration. Even closely related species may have different migration strategies. The study compared the activity patterns and fuelling of lab-held warblers in southern Sweden during autumn migration, and found differences in their migratory strategies and species-specific adaptations to magnetic field information.